Activation

I Bought An IPhone 5s & Idk why But It Won’t Let Me Activate It With Any SIM. It Says That The Carrier Doesn’t Support It But I Have No Idea. I Went To Sprint And Boost And They Told Me It Was Locked. How Can I Unlock It Or Make It Accept Any SIM ? I Tried Reaching The Person But They Don’t Reach Me Back. Can You Please Help ? I Honestly Have No Idea What To Do ?