Can I place a sim card in my at&t iPhone 5?

I bought my iPhone 5 with a plan from at&t, so I have to pay monthly you know?
So i though why don't i just take out the at&t Sim card and place a pre-paid sim card. Would it still work?
I was just wondering:-O

Also, would my phone be considered to be "locked"? So I wouldn't be able to change it?

1.It will most likely be locked to AT&T.
2. If you got the phone recently you may be under contact with AT&T and have to pay a cancellation fee.
3. If you go with prepaid, you should probable get one associated with the AT&T network, it may not be compatible with the high speed data frequency on other networks.