It's and its sound alike, but their meanings are quite different. It is not uncommon to confuse the two, but your English will confuse other people if you use the wrong one!
It's is a contraction for it is (or it has). As an example, "It's a beautiful day today!", or, "I think that it's time to go." This is a very common contraction to hear in spoken English.
Its is a possessive adjective. It modifies a noun by telling us to whom it belongs. For example, "The cat is washing its paw," or, "The company hired its first worker." Other possessive adjectives include my, your, his, her, our, and their.
The rule to remember is this: If you mean "It is" or "It has," use it's with an apostrophe ('). If not, don't use the apostrophe.
It's vs. Its Grammar Quiz
It's vs. Its Lessons: