No pair is confused more often than it’s and its (and the similar pairs they’re/their and who’s/whose). This is probably because English usually uses an apostrophe to indicate possession (the boy’s book), but in these pairs the rule is the other way around:
it’s is an abbreviation for “it is”.
its means “belonging to it”.

The easiest way to remember this is that it forms a consistent pattern with other pronouns and abbreviations:

he’s, she’s, it’s, they’re, we’re, who’s are all abbreviations.
his, hers, its, their, our, whose are all possessives.

So, if in doubt, ask, “Can I substitute ‘he’s’ or ‘his’ for this word?” Then use an apostrophe if the answer is “he’s” and don’t use one if the answer is “his”.