[RonBee]

What Mike said is exactly right. I will elaborate on it just a little.

Its is, like his and hers, a possessive pronoun. None of them ever ever ever take an apostrophe. Never. It's is a contraction for it is or, occasionally, it has. Most of the time, it's stands for it is, but occasionally it stands for it has. Context is the key for deciding which it should be.

It's been a good day = It has been a good day
It's been a long time = It has been a long time

More often, of course, it's = it is.

It's something.
It's nothing.
It's a good thing.
It's a bad thing.
It's a sad thing.
It's a wrong thing.
It's a strong thing.
It's a crude thing.
It's a rude thing.
It's a food thing.
It's a mood thing.
It's a diamond ring.
It's a turkey wing.
It's a mockingbird.
It's a word.
It's what it is.