Never fear; it's all a lot simpler than it seems!
It's = it is / it has
The apostrophe always shows that a letter is missing; in this case, the "i" of is or the "ha" of has. It's also called an apostrophe of omission.
It's (it has) been a long day.
It's (it is) good to know you're back home.
Its = "belongs to it"
This is always the possessive form of the word, as in:
Her research √
His computer √
Its conclusion √
When using pronouns, you would never write he'r research or hi's computer – so why would you write it's conclusion?
Finally, if you're tempted to try out your's, her's, our's, or their's, the same rule applies because they're all pronouns. You wouldn't write the bed is hi's, so you shouldn't write the bed is your's or their's—or it's.
Keep this apostrophe for nouns only: people, animals, or objects!
Noun: Janita's hat √
Pronoun: her hat √
Noun: Rover's collar √
Pronoun: his collar √
Noun: the cat's whiskers √
Pronoun: its whiskers √
Noun: The train's windows √
Pronoun: its windows √
To summarise:
it's = letters are missing from a word or words
its = something belongs to 'it'
Pronouns (mine, yours, his, hers, its, ours, theirs) don't have an apostrophe of possession, but nouns (people, animals, and objects) do.
Let me know what else helps you remember the difference.