As I came across learning ga while learning japanese, I seem to be having allot of trouble with the diffrence it has from wa. I later discovered that I wasn't the only one, not even close. After reading a few notes from people or from sites, everyone had a diffrent explaining and this got me interested. I would like to have a discussion about the particle ga and wa, I would like to see from many people on how they separate it. Matter of fact, I would like share mine; wa and ga can be used after the topic. And as much as i am bad at explaining things, ga is more like a bully. 私は学生ですwatashiwa(rarley used)gakuseidesu-I am a student, out of all of the people here i am a sudent, i am one of them. but 私が学生ですwatshiga(also rarley used)gakuseidesu-would also mean "I am a student" but your the topic, your the one being given attention.
が doesn't go on the topic of the sentence, it goes on the subject of the sentence. If you see が, that means that thing is NOT the topic. It's only the thing doing the action.
For that reason, が is always used for "unknowns" and new information and never は. は marks "what we're currently talking about", and it's hard to say "we're talking about (a specific person that I don't know)". Anyway, it means that you can only use は with things that both parties in the conversation are aware of.
There's far more to it but it is very importent for you to understand the difference between the topic and the subject of a sentence.
は is used when the topic is obvious or has already been established. For example, if you're meeting someone and your saying your greetings;
私は (name, a student etc.).
が, then, is the opposite - it's used when introducing a new topic. For example, if someone asks you to pick a fruit from a basket;
ま、りんごが好きけど、今は、バナナをいたいです - Well, I do like apples, but right now I want a banana. (informal language)
が can also be used for emphasis, when you're angry or amazed or want to demonstrate some emotion;
あのくそものが。。。 - Those bastards...!
Hope that helped :)