its or it's, your or you're

It's is short for it is or it has: it's late, it's been a long day. Its, with no apostrophe, shows possession: the dog wagged its tail.

You're is short for you are: you're right. Your shows possession: your bag. The same pattern holds for who's (who is) and whose (belonging to whom).

To check, read the long form aloud: if it is or you are fits, write the apostrophe. Possessive pronouns never take one: its, yours, hers, ours, theirs.