Can i use my old at&t iPhone 4s on Virgin Mobile instead of buying a new one?

Can i use my old at&t iPhone 4s instead of buying a new iPhone 4s from virgin mobile themselves?

No you will not be able to. You will need to buy a Virgin Mobile iPhone since AT&T is GSM (uses SIM cards) and Virgin Mobile is CDMA (uses Satellite for service/No SIM cards). You can probably try to trade someone on Craigslist or sell it on eBay and use the money towards a Virgin Mobile iPhone