“It's” is the contracted form of “It is” or “It has”.
This form is used in sentences using “it” as the subject of the sentence with the verb “to be” used as either the helping verb (e.g. It's going ..., It's raining ...) or the main verb in the sentence.
It’s difficult to find work these days.
It’s going to rain soon.
“Its” is the possessive pronoun form.
This form is used to express that “it” has a specific quality, or that something belongs to “it”.
I found its taste to be superb!
Its color is deep red, almost Burgundy.