Have an iPhone 5c with no monthly plan with US cellular

[iMore Question]

What will I need to do to get AT&T as My carrier under my family plan?

 

[Rob Phillips]

As long as the 5c is unlocked you should be fine. Contact AT&T to start service and initiate the porting process, or just take the phone into an AT&T store and I'm sure they'll take care of it for you.