I found an iphone 4s from att on the sidewalk?

Can i still put service in it if its reported stolen?

No once it's reported stolen you can't put service on it. You can basically only use it as an iPod after it's reported stolen

No. Return it to the closest AT&T store or call some of the numbers on the phone to find out who it belongs to so you can return it to the rightful owner. If you return it to the owner, you may get a reward.