저,내가, 나-- they all mean "I", but 저 is a formal way of saying 나. For example, when you're in a formal situation or speaking to someone older than you, you would say: 저 벌써 밥 먹었어요. (I already ate.) When you are talking to your friends, you would say: 나 벌써 밥 먹었어. (I already ate.) This one is kind of tricky to explain as it can be used in various ways. The two most common ways 내가 is used are: 1. combination of I & (helping verb) 내가 빨래할게. (I will do the laundry). 2. my 내가 제일 좋아하는 가수. (my favorite singer) Hope that helps!