HELP! How do I get an iPhone?

OK, so my grandparents signed up with AT&T in January, and got two free phones. Well, as I didn't have a phone at the time, they gave one of the two to me. I sold the free phone, and bought a Samsung BlackJack II. Just recently, I sold the BlackJack II, and now, I really want an iPhone. All I have right now is a SIM card. My grandparents didn't sign up for insurance, so I can't get another of the free phones. Can I upgrade to an iPhone through AT&T? How would I go about getting an iPhone?