Its Vs It's

It’s is a contraction of “it is” or “it has.”
Its is a possessive determiner we use to say that something belongs to or refers to something.

It’s and its are among the most commonly confused words. They are pronounced the same, there’s a very small difference in how they’re written, and it’s also easy to mistake the contraction in it’s for a possessive. But the rules are very clear — it’s is the same type of contraction as “where’s” or “there’s,” and its is a possessive just like “my” or “your.”

Examples

He doesn’t think its a good idea to spend all his money on video games.
He doesn’t think it’s (it is) a good idea to spend all his money on video games.

Its been a year since the last time they met. 
It’s (It has) been a year since the last time they met.

The computer looked as though it’s power supply had failed.
The computer looked as though its power supply had failed.

A car is no good without it’s wheels.
A car is no good without its wheels.