Here is a handy homonym sheet for your use.
While an apostrophe usually shows possession making it seem logical that if something belonged to it that it would be spelled it’s. HOWEVER it’s not the case, so I’ve come up with an easy way to remember which way is right. An apostrophe also combines 2 words.
So, if you can say it is.. and the sentence makes sense, it gets an apostrophe.
I know it’s hard… = I know it is hard. CORRECT!
The bottle was on it’s side. = The bottle was on it is side. WRONG.
I have problems with this one. Of course, I also answered on a test that the contraction for ‘will not’ was ‘willn’t’ (logically speaking it should be). Grammar is not my friend. 😦
Hope this helps!