は is kind of like the English 'a/an', while が is like the English 'the'. 私は学生です。I am a student. (As for me, I am a student) 私が学生です。I am the student. (I am the one that's the student)

This is good question because actually this is one of the topic we discussed how to use. basically は is more common and used often. I'll talk about situation. 私はご飯を食べる。 私がご飯を食べる。 In first one with は, this means "I" will eat rice. I don't know somebody else will eat. In second one with が, this means I will chose to eat rice. So somebody else can't choose rice. So for example when you order something in restaurant, you basically need to use はbecause your friend's order doesn't matter with your order. However, when you eat dessert, you find set menu which suggests you a couple of different flavor ice for share such as vanilla and chocolate. 私がチョコレートアイスを食べる。 In this case, your friend can't eat chocolate ice. But finally if you don't order set one. 私はチョコレートアイスを食べる。 In this case, your friend can eat chocolate ice because they order separately. That's different.

this is not the full answer, but the quick simple rule is: は for (general) topic ("talking of yesterday....") が for direct subject ("yesterday was...")