Its vs. It's

Here is the entire rule: it's is short for it is or it has; its (no apostrophe) shows possession. That's it. The confusion comes from the fact that apostrophes usually signal possession (the dog's tail), so it's looks possessive. It isn't.

The two-second test

Read the sentence out loud with it is (or it has) in place of the word. If the sentence still makes sense, write it's. If it sounds broken, write its.

“It is been a long day” → awkward but it works as it has: “It has been a long day” → it's. “The dog wagged it is tail” → nonsense → its.

Why possessive its has no apostrophe

Possessive pronouns never take apostrophes: his, hers, ours, yours, theirs – and its. The apostrophe in it's isn't marking possession at all; it is standing in for the missing letters of is or has, exactly like the apostrophe in don't or she's.

One more thing: there is no its'

Its' (apostrophe after the s) is never correct in modern English. If you ever type it, one of the two forms above is what you meant.