What do I need to activate an iPhone 3GS bought from Craigslist?

I've never owned an iPhone. I have this guy that wants to trade me an iPhone 3GS, but I've never been on a contract plan, I usually get Virgin Mobile, but I broke my phone over the holidays. So what exactly should I ask to make sure I can activate this phone? And if everything is legit, and I buy the phone, what do I need to do to activate it? Virgin mobile also does a thing know where they sell iPhones, is there a chance I could just use this phone with their pay-as-you-go plans?

Added (1). If I need a sim card, how do I go about getting one? Do I just bring the phone to a cell-carrier, like Verizon?

You can't use that phone with Virgin.

Virgin is a CDMA carrier and that 3GS is a GSM device that requires a SIM card.

AT&T is your only option unless the phone is unlocked. If unlocked, T-mobile is an option too. Be sure to get the ESN number from the seller. Call the wireless company where you plan to get service and make sure the ESN is clean. The service provider should give you a sim at low or no cost.

Used iPhones, especially ones as old as the 3GS, are notorious for having dying batteries. Be prepared to pay for a battery replacement as part of this deal.

You'd be better off keeping an eye open for an iPhone 4 if you want good value and a good bargain. The 3GS is a bit behind the curve now.