It’s – a contraction for it is or it has
Its – the possessive form of it

A foolproof way to determine which to use is to attempt to break the word down from the contraction form. If “it is” fits correctly into your sentence, use the contraction – it’s.

An example of their usage is:

It’s (it is) going to be a busy day for the office and all of its (possessive of it) employees.