I'm sick of seeing people write "it's" because 999999 times out of that-plus-one they're using it incorrectly.
It's simple. Even a Magic-playing moron like you should be able to understand.
IT + IS fused together make IT'S.
The apostrophe--the "sky comma"...ugh, the "floaty period-with-a-tail"--tells you words "fused" here.
ITS means possessive.
Just, whatever you'd normally do whenever you see the letters I, T, and S together, do the opposite.