how to use an unactivated 5c to att?

Well did you not try to put your sim in already?

 

First where did you purchase it from? Second what carrier is it?

 

Should be fine I would make sure there is no activation lock on it.

 

Activation lock is a security feature with IOS 7. If the phone is restore, it requires the original users iCloud password to unlock it, otherwise it will be a paperweight and can't be used.

http://support.apple.com/kb/PH13695

 

not entirely true though. It could have been activated on a line then deactivated and restored with the apple id still logged in.

 

If you pop in your SIM and it activates without asking for an iCloud password, you're good.

Otherwise you need to get the latter from the seller or get a refund.

 

Not if its Activation Locked. If it is restored without the original owner signing out of iCloud, then it will be locked to iCloud unless/until you enter that specific iCloud password.