I need cell fone help...?

We bought a cell phone from a guy off line and its a Sprint black berry...we're trying the cheapest way to turn it on and get it up and running without a costly bill...we're only 17 so we cant even legally get a contract..but we just need help to get the phone working...any tricks or extremely cheap methods will work and be very much appreciated...(also to let you get a better understanding of our situation the phone isn't even activated yet right out the box..new phone)