Subjective pronouns are used as the subjects of sentences (whom or what you're talking about).
I am going to leave for my appointment.
She is late already.
They will never make it on time.
A problem occasionally arises when subjects are compound. You might read, for instance:
His brothers and him are going to the ball game.
Margaret, Elizabeth, and me were at the mall for four hours yesterday.
Me and her see eye-to-eye on lots of things.
These pronouns are used incorrectly. Because the pronouns are used as subjects of the sentence, they should all be in the subjective case: I, you, he, she, it, we, or they. So, the sentences should read:
His brothers and he are going to the ball game.
Margaret, Elizabeth, and I were at the mall for four hours yesterday.
I and she see eye-to-eye on lots of things.
(It's considered polite to put the other person first, so it's better to word this sentence like this: She and I see eye-to-eye on lots of things.)
If you're not sure if you've used the right pronoun, try writing or saying the sentence with only one subject. You'd never say:
Him is going to the ball game.
Me was at the mall for four hours yesterday.
Change the pronouns to the ones you'd normally use when there's just one subject (he and I).
Source: The Only Grammar Book You'll Ever Need