Even native English speakers make mistakes with its and it’s, even though it’s really not difficult to remember.
Its is a possessive pronoun.
Example: The dog carried its ball in its mouth.
It’s is a contraction of it is or it has.
Example: It’s raining.
Probably part of the confusion is that we do use apostrophes for possession (for example, the dog’s ball), but we use them with nouns. When we use possessive pronouns, we don’t use apostrophes.
It might help you to remember if you think that the other possessive pronouns for the third person singular (hers and his) don’t use apostrophes.
It’s easy to check yourself. Whenever you write it’s, say it without the contraction to check that it’s correct.
So, it is easy to check yourself… say it without the contraction to check that it is correct!