A lot of people find this one confusing and, quite honestly, a lot of native English speakers people mix them up!
“It’s” means “it is“, as in the sentence: “It’s a great job”. Remember that the apostrophe is there to show you that the ‘i’ has been removed!
“Its” is a possessive pronoun (sorry, that it’s a bit technical!), as in the sentence: “This company is known for its great customer service”.