Welcome to Gaia! ::

Normally I would lock threads like this and point to the sticky right above, but since this is your first time here(at least as far as I know) I will let it remain open.

As for your answer, there are many uses for the particle 'ga', a lot of which I will now try and encapsulate.

The first use they often teach teach you is with expressing existence, such as how we say "Somethin's here". The basic sentence structure for these kinds of thoughts is "○がいる/ある" The short version of what separates いる & あるis that ある is used when referring to non living things(like bikes or buildings) while いる refers to what's living. This structure is also used when asking if something or someone is there.

Another very important and often overlooked use of the が particle is it's use with verbs. A while back I wrote a lesson stating を(when used as a particle is read as o) is the Direct Object Particle, and that is still true; however that particle is used with Transitive verbs(meaning the action was performed by an outside force). The が particle is used with Intransitive verbs(meaning the action was performed without outside force compelling it to do so).

An example of the difference would be if your computer or TV suddenly turned off without you doing anything. If you find that you knocked the plug from the wall somehow, that means an outside force cut the power, which of course is transitive. Intransitive would be if the plug was still in and everything else was still on; at that point I'd call either an electrician or an exorcist, perhaps both if weirder crap came about.

The most common advanced use of が would be as a sentence connector. The use they teach you first is creating a contrast, like when we say "This guy's poor but happy." (この人は貧乏(びんぼ)が嬉しいだ。) or "I read it but Yuko didn't read it." (読みますがゆこさんを読みません。)

The second, less often taught use is asking favors(such as with the the classic line 失礼(しつれい)しますが[excuse me but...{could you help me?}]). This is also a nice way to add some politeness and take the edge off whatever you're saying, such as if you wanted to ask to borrow your boss's pen or a stranger's phone to call home base and say you lost all your money at Pachinko and need a ride home

A lesser known use is asking extending an invitation to someone. For example, say you wanna ask a friend to see ACTION MAN 2: ACTION MAN USES CARS, a movie you've been itching to see since the first shots leaked to the internet. To ask the friend in question you'd say: Action Man 2を見に行きますが、 一緒に行いかいのですか? "I'm going to see Action Man 2, you wanna go along with me?"

Just doin' my job, ma'am