It's simple. when you use "が", the emphasis of the sentence will be on subject. For instance, 彼女は車を運転します。　＝＞　She drives a car. 彼女が車を運転します。　＝＞　SHE drives a car. When you use "が", it will mean like " She drives this car, not someone else. " Thus, unless you want to put emphasis on subject, you could usually use "は" as a subject marker.

Let me add a little to poisonous apple's comment which is accurate though. You also use が　in relative clauses as well as in comparison. This is the chocolate I bought. これが/は私　が　買ったチョコレートです。 You can do it better than me. あなた　が　やったほうがよい。　あなたのほうがじょうず。 Also, when you make your own choice. I like this. これ　が　いい。　　If you say これ は いい、it means "This is good," and it's more of a general statement than expressing your preference. 　Study this. 　 アメリカ　は　いい国だ。　でも私は　日本　が　いい。