Now! On to the usefulness!
Question: Alright, so I want to play guitar, but I know NOTHING! Tell me things!
Alright. So, since you haven't bothered to research too much beyond finding this Musician's Forum on Gaia, you might not be the most determined guy around. It isn't too hard to type in "basic guitar tutorial" on Google and find some results that are useful within five minutes. As such, you might not be too determined to do too much with your instrument once you get it, and if you do, then you can eventually upgrade beyond the absolute basic: an acoustic starter pack.
Some people want to start out on electric, but I'm pretty sure that no guitar-wielding music veteran would recommend this. In fact, they would recommend an acoustic. An acoustic is more physically intense in terms of how hard you have to push the strings down and, quite frankly, you're not going to know s**t about how to make an electric guitar tone that doesn't sound like a slowly deflating a** so an acoustic will sound better to your ears. Since you have NOTHING in terms of guitar knowledge, you should just accept what is said here at base fact when I say that you have three options in terms of an acoustic starter pack: the cheapest Squier start pack, the more expensive $150 one, and the Fender $200. If you have the money to spend and feel comfortable with your determination in learning the guitar, go with the Fender - it sounds better, plays better, and lasts longer. If not, either the nicer $150 Squier pack or the $100. You can find all three of these at your local music shop in all likeliness. You will also want to buy a chromatic tuner for $20. Unless you have previous musical experience and have a developed musical ear, you will need it until you become familiar with the notes of a guitar and can ear-tune. Get some picks, too, if they're not offered to you. They're little plastic wedges that most people use to play the guitar. Some people use their fingers, which is fine, but this guide is going to act like you're using a pick because finger-picking is thing you learn through someone showing you.
Question: Alright, so now I've got my guitar, but how do I play it? More importantly, what is all of this s**t on it? Its got like pieces of plastic on it and and knobs and s**t and its like ******** rocket science, man.
Well, first off, I'd like to extend my happiness to your chemistry professor for flunking you so you can never become a rocket scientist and ******** NASA in the a** by telling them calculus is as hard as the part's of the guitar. Fortunately, there are idiot-proof guides to the parts of a guitar. These are called pictures with labels.

Congratulations. You now know the parts of an acoustic guitar.
Question: Alright smartass, I tried to strum my guitar, but it sounded like s**t, and I thought it was out of tune, so I plugged it up to the chromatic tuner you were raving about, but I have no ******** idea what all of these letters mean!
You mean those ones that are A, B, C, D, E, F, and G? Those are the notes in the musical alphabet. Sometimes you will see A# or C# or something like that and those are the notes in between the notes. An A# is the note between A and B, and depending on your tuner, you might also see a Bb, which is the same thing as A# but with a different name. Since you now know what those letters mean, you should know that your guitar tunes from the sixth string to the first string (the thickest string to the thinnest string) the notes E, A, D, G, B, and then E again. Since you're a newbie, I suggest you tune all of your strings down to where they're sort of loose and make a jangly noise when you pick them, and then tune them up accordingly. Many a newbie has made the mistake of tuning too high and realizing they were tuning up to an E that was an octave (a lower E to a higher E or a lower A to a higher A and so on) higher than the one they're supposed to be on and then the string snaps.
Question: Oh s**t, I didn't read that last sentence until AFTER I broke a string. How do I fix it? Do I have to buy a new guitar?
No, you don't. That's one of the beauties of the instrument. You just go to your local music shop and tell them you want a pack of strings for the guitar. They'll ask a question like, "What kind? What gauge?" or realize that, since your description was so vague, that you're a newbie and pick a pair of good, cheap strings that are a low gauge. What's a gauge, you ask? Its how thick a string is. Since you're new, you'll want to start out about a 10 gauge thickness for your strings - the middleman of thickness, in between the ridiculously low 8 gauge and the rope-like 13 gauge. It'll cost you about $8 or so dollars for a pack of strings, but don't b***h. Guitar strings are cheap compared to bass strings. While you're there, you're going to want to pick up a string cutter and a string winder to make the job easier. There's one they sell that's both in one. The guys at the shop will most likely point you to this and quote you anywhere from $10 to $20 on it, depending on their prices.
Alright, so you're home, you've got your pack of strings, and your d**k is in your hand from the anticipation. You want to find the same string you broke on your guitar in your string package. It isn't hard to figure out how the individual package labeling works out, but just in case you can't figure it out, it goes from thinnest string to thickest. Those numbers are the thickness of the strings in a numerical measurement.
You've found the one to replace. Great. Unwind what's left from the string you broke out of your machine head, then pop off the bridge pin that has the broken part of the string in it and throw that broken string away. Now take the end of your new string that has the ball end and stick it in the hole, then put the bridge pin in. Make sure the string falls into the groove of the bridge pin. Now stretch the string out to the machine head and string it through, but only after you put your fist on the neck at about the fourth fret and ran the string over that. Now remove your fist and start tuning the machine head in the same direction as the rest of the ones on the side of the headstock with your string winder. Watch the bridge pin. It might come out.
Now tune up to the note of the string, then go to whatever fret you want and use a few fingers to "bend" the string as high as you can. You'll notice that, when you pluck it, your string's tuning went down a bit. This is expected. Tune it back up and repeat the process until, when you bend the string, it doesn't go out of tune at all.
Question: Wow man, thanks! Can I have your babies? I mean, what do I do next?
Well, if you fixed the string you broke or you didn't break one and your guitar is perfectly tuned now, you're ready to start playing. If you can afford it, there is one suggestion that is king above all in learning guitar: taking lessons. Find a competent teacher (ask the guitar players in your school who learned how to play from a teacher for their opinions on a good one, but this is if you're in school). This may be anyone from a professional to a friend who's willing to offer free lessons. Either way, this is the best way to learn and you no longer require this guide if you get a teacher.
But if you can't afford a teacher, well then! You have to rely on online tutorials and my smarmy mouth! Ho-ho!
The first thing you learn: the open string chords. A lot of guitarists don't get past these chords in terms of playing, but that's fine! They're hobbyists. Its nothing to be ashamed of, man. At least you can say that you were determined enough to get THIS far! Hobbyists will be hobbyists and serious guitar players will be serious guitar players, but only if they have the talent and skill for it.
These are your open string chords:

Alright, so you're going, "What the ******** does this s**t mean?"
Once I explain it to you, you're going to think, "D'oh!"
So, let's take the C Major chord for example. You see a box. That is called a chord chart. This is how chords are presented to most guitarists, though some use sheet music to read them, but that's beyond the point. The box is divided into grids. The grid lines going straight down are your strings in the order from the sixth (farthest left) to the first (farthest right). Just above those grid lines going straight down, you may see an 'o' or an 'x'. If you see an 'x,' then you don't play that string or, when you get to more advanced chords, you mute that string. The 'o' means that the string is open, which means you don't fret any notes. The grid lines going across are representations of your actual frets. In between those is where the notes will be. So on the 2nd string on the C Major chord, you will see there is a green circle on the first row. That means you put your finger there, right on the first fret of the B (2nd) string.
Get it? Simple, huh?
Yeah man! I totally got it! And I practiced it and I got this stuff and decided I wanted to try to find some songs, but how do I do that?
Well, other than learning by ear, you can go to websites like ultimate-guitar.com. These websites have tablature, which is essentially a specialized sheet music for guitar, but its not sheet music. Its the easiest thing in the world to read next to books in your native language. I don't even need a picture to explain it properly. Tablature is six horizontal lines going across that represent your strings, with your thickest E string as the bottom tablature line and your thinnest E string as the top one. Looking at those lines, you will see numbers on it, and you're thinking "what are these numbers?" Well, those numbers represent frets on that particular string. So a 3 on the top line means you play your 3rd fret on the first string. A bunch of numbers lined up is a chord, and you might be able to recognize some of them just by looking at them after a while of playing.
On tablature, though, there are sometimes special symbols. These will most likely be explained in a legend as what they mean, as some people like to vary how they represent these guitar techniques, but here are the basic ones (there are others).
Hammer-on: You pick a note, let's say the third fret on the 1st string, and then you press a finger down on, let's say the fifth fret on the 1st string, right after you pick that note. That is hammering on a note. Sometimes people hammer on a fretted note after picking an open note. This is harder to do and takes practice.
Pull-off: The b*****d inbred cousin of a hammer-on. You line up two notes, let's say the third fret and fifth fret again on the same string, and you pick while having one finger on the fifth fret and another on the third fret. Then, right after you pick, you pull your finger off the fifth fret and keep the other one on the third fret. That's pulling off to another note.
Slide: You just pick a note and then slide your finger up the fretboard to a desired note. This may not sound good at first, but push down harder than you normally would when doing it at first, then gradually ease up as you get stronger.
Harmonic: Just lay your finger across the right metal fret of whatever number it tells you to. These are most commonly the 12th, 5th, and 7th frets.
Bend: This is hardest on an acoustic. You pick a string, and then you bend it up with your finger(s) like I talked about earlier during tuning. Most people use more than one finger to do this, so don't feel bad if you have to. These come in the flavors of a full bend, a half step band, a quarter step bend, a two step bend, or even two and a half and three step bends. A full step is one note higher than the note you started out on. A half step above A would be A# and a full step would be B and so on.
Vibrato: Every stringed instrument does this. You basically do what you do with a bend, but you do really small bends, like quarter bends, and basicall shake the string at a speed. Most people do them fast, but for like a slow ballad, slower is good, too.
Palm Muting: You take part of your palm and lay it against your strings at the bridge of your guitar and strum. Angle your palm so that only the side of it touches. You can increase how heavy or light the pressure is depending on what sound you want to get.
Muting: Take your left hand and lay it against the strings. This creates a percussive sound. Advanced guitar players will use chords that have strings muted by angling their fingers, but you're not there yet.
With this basic knowledge, you can teach yourself the rest and possibly even teach yourself music theory. If not, then maybe one of the other guys here will be kind enough to post a thread teaching basic music theory. I only know basic music theory (scales, staying in key, modes, constructing chords) that applies to guitar, but!
If you have any questions, post them and they will be answered.