Two words.  Three letters.  Totally different meanings.

Its without an apostrophe is a possessive, like his and hers, ours and theirs.  If something is showing ownership, then use its: The tree dropped its leaves in the autumn.

It’s with an apostrophe is a contraction of two words: it is or it has.  The apostrophe takes the place of the missing letters; read over the sentence and mentally fill in the missing words to ensure that you are using the correct form: It’s been a long day. It’s going to rain.

Contractions such as it’s should be avoided in academic or more formal writing.