Iphone 3gs reads no service?

Ok so I have thise iphone 3gs lying around I wanna fix it but regardless what sim card I put in it… It always reads no service I tried down grading it to the 5.1 firmware but nothing happened. Can someone help me please

You have to take it to a store and have it activated. Iphone 3GS had no way to get to sim card by the way. That was the 4s.

You have to go to a store to get an activated sim card