www.musictheory.net
Serious - great site for learning music theory.
As for hearing a note and knowing what it is - unless you put your ears through ear-training boot camp followed by ear training Navy training, you're probably not going to be able to do that. Knowing a pitch based entirely on just hearing it is called 'Perfect Pitch,' (or Absolute pitch.) It's just a thing that some people have. What is more generally attainable is 'Relative Pitch.' That's the ability to determine what a pitch is based on other pitches.
That'll go for chords to - you'll be able to tell if it's a V7, or a vvi7 or a major chord, but unless someone gives you a frame of reference, it is unlikely that you'll be able to tell what the starting note is.
I'm surprised you can't already figure out which songs are in major and which are in minor - usually people describe minor songs as 'sad.' (Which isn't necessarily true...) Typically, Hebrew songs are in minor liiiiike Havah Nagilah
Some well known minor songs:
Greensleeves, We Three Kings of Orient Are, Joshua Fit the Battle of Jericho, House of the Rising Sun etc.
Some well known major songs:
Basically everything ever written. Twinkle twinkle little star, Alouette, This Old Man (i'm stuck in nursery rhyme mode now.) Frere Jacques...
Ooo! Actually, that's perfect.
Listen to Frere Jacques. (try not to gag at the wiggles...)
Now listen to this!
Aside from probably not making you want to gag, and the second one being played with an orchestra, can you hear the musical difference?
The first one is major, and in the second one, Mahler altered the pitches to make it minor.