How to put new SIM into iPhone 5? Activating it?

I just upgraded to a 5 at Target, and I was told that I have to put a new SIM card into the phone before I can do anything with it. The card though, is too large for the slot in the phone. I've looked into it online and it's possible for me to "cut down" the card into a nano size, or whatever is necessary, but I've just been scared to mess the phone up.
I'm thinking of just going back to Target tomorrow and let them fix it, because I was pretty sure they should have activated my phone there with the new SIM. Does anyone know if I can do this from home or should I just take it back?

Take it back

The iPhone 5 usually comes boxed with a nano SIM already in the phone, the SIM card number is located on the box. I'm not sure why this would not be the case for you unless the employee at Target removed it for some reason. I have seen people cut a standard SIM to micro SIM size, but people usually fail at this. I have never seen someone successfully cut a standard or micro SIM to nano size, there just isn't enough "casing" to cut it without damaging the card. You would need some very fine tools and a very steady hand. You can return to Target to get a SIM or go to your carrier's store. It should be Target that rectifies the problem though, since they neither activated the phone nor provided you with the means to do so yourself.

They should have cut it, but it's pretty easy to cut a sim yourself. They can't activate phones there other than ones that are not smartphones based on the fact they just sell the phones, if i remember correctly. You should be able to do it from home, it was an easy task with my iPhone 4s.