Its without an apostrophe refers to a possessive pronoun ‘it.’ It’s with the apostrophe is a construction for the ‘it is.’ Even if you are clear with this difference, if you still can’t use these properly, then try to replace the verb ‘is’ in the ‘it is’ with ‘has’ to sound like ‘it has.’ If it fits the sentence, then you are free to use ‘it is.’ Otherwise, use ‘its.’