The word it’s only has an apostrophe when it is a contraction of it is or it has.
It’s a lovely day.
The film was poor, I’m glad it’s finished.
Now I can ride a bike, it’s easy!
Apostrophes are used to show possession, but there are other ways to show possession using possessive pronouns or determiners:
Possessive pronouns: mine, yours, his, hers, its, our, theirs.
Determiners: my, your, his, her, its, our, their.
These words do give an indication of belonging or possession, and some of them end in -s, but they never have an apostrophe.
So, if you are not sure whether to write its or it’s, simply substitute the words ”it is“. As a rule of thumb, if the sentence still makes sense then you can still use the apostrophe, otherwise leave it out.
Generally apostrophes are never used to form plurals. However, it is acceptable to use an apostrophe for the sake of clarity when forming the plural of a single letter or a single number:
Find all the number 7’s on this page.
Mind your p’s and q’s.
Put a circle round all the t’s in tittle-tattle.