*its
"it's" = "it is"
"it's" = "it is"
The rule is, in English, you can make a noun possessive by adding "'s" to the end.
Let me emphasize one part: that's the rule to make A NOUN possessive. "It" is a pronoun, not a noun! The rule doesn't apply any more than it applies to a verb or an adjective.
Hence you don't have stuff like "you's", "he's", "she's", "we's", or "I's", and you also don't have "it's".