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.