Switched the sims card to a different iPhone 4 s

I had my sims card in an Iphone4s. It worked fine. I got a better looking Iphone 4s so I put the sims card in that phone and now it says no service and I don't know how to activate it without Wifi connection. It says there's no network.What can I do to fix this problem. Please help me. Thnak yall very much.