[qwikz28]

It's is a contraction for it is or it has.

Its is a possessive pronoun meaning, more or less, of it or belonging to it.

And there is absolutely, positively, no such word as its'.

A simple test

If you can replace it[']s in your sentence with it is or it has, then your word is it's; otherwise, your word is its.

Another test

Its is the neuter version of his and her. Try plugging her into your sentence where you think its belongs. If the sentence still works grammatically (if not logically) then your word is indeed its.

Examples

It's been good to know you. Contraction: it has
It's a bird! It's a plane! Contraction: it is

The dodo bird is known for its inability to fly. Possessive pronoun: its inability = the dodo bird's inability