IT'S is the shortened version for IT IS or IT HAS. Examples using IT'S:
It's very cold outside today.
It's so nice to meet you.
Did you hear it's going to rain?
It's been a long time time since I have seen her.
ITS is a posessive pronoun. Examples using ITS:
The dog ate its bone.
I love Norway for its beautiful mountains.
The company is going bankrupt. Its employees are scared they'll lose their jobs.