I see these homophones used incorrectly over and over (and over). Here are some quick tips to get it right.

Its
This is relating to itself as the owner/possessor. Whatever it is, it belongs to it. (The dog is gnawing its bone.)

It's
This is a contraction for "it is" or "it has." (It's time to water the flowers.)

Their
This is a plural pronoun. Whatever it is, it belongs to them. (The girls put their books on the table.)

They're
This is a contraction for "they are." (They're going to get cookies.)

There
This is an adverb that means in that place. It can also be a noun that means that place. (Go and stand over there.)