How to ACTIVATE unlocked iphone? (DETAILS BELOW)?

So LONG story shirt my android SPRINT moto x got stepped on and the screen is complete ruined so my brother gave me his old unlocked iphone 5c. I put the Nano Sim card from my motox into the iphone 5c (that didn't have a sim card when he gave it to me) it fit in perfectly but it doesn't let me activate it. It says ACTIVATION ERROR ACTIVATION SEVER DOWN or something like that. Won't even let me activate it with my itunes account. I CAN USE THE PHONE ON Wi-Fi WITHOUT SIM CARD but the problem is i have no actual data service to send texts etc.
should i take it to the sprint store (my carrier) and let them activate it? Or is there another way? Please tell me any tips and tricks you may know because i'm pretty much desperate.

The phone probably is not unlocked. It may be locked to a certain carrier. It's either that or there's a problem with the SIM card.