How to find network provider on IPhone 5?

I have recently been given an iPhone 5 from a friend. She acquired the phone as broken from an ex-partner and was planning on fixing it and selling it on but never did. When she discovered I was in need of a new phone she gave it to me. I got the screen replaced as it was hanging off. I have done a factory reset on the phone and to set it up it is asking for me to insert a sim. I have tried inserting a sim but it says

"the SIM card that you currently have installed in this iPhone is form a carrier that is not supported under the activation policy that is currently assigned by the activation server. This is not a hardware issue with the iPhone. Please insert another SIM card from supported carrier or request that this iPhone be unlocked by your carrier"

How do I find out what network the phone is locked to? I have never had an iphone before so I don't really know what I'm doing to be honest. I just assumed it was unlocked. I can't seem to get past the set up stage without putting a sim in.

I suggest you go to a local Apple store then have them help you.

http://www.imei.info/check-warranty-unlock/
IMEI should be on the back of the iPhone itself or the box