It’s is a contraction of “it is” or “it has.”

It’s time go home.

Do you think it’s ready?

I corrected your homework – it’s very good.

Do you know where the checklist is? It’s on the table.

It’s been a long time.

Its is the possessive form of “it.”

What is this instrument – what is its purpose?

The A380 is making its take-off roll.

This aircraft has a mind of its own!!!

The bird lost some of its feather  

The confusion between it’s and its occurs because on virtually every other word ‘s indicates possession, so English speakers naturally want to use it’s to mean “something belonging to it.” But it’s is only used when it’s a contraction of it is or it has.

The ironclad rule – no exceptions – is that if you can replace the word with “it is” or “it has,” use it’s. Otherwise, it’s always its.