> just place the sim card there, and close it up, right?
Normally you can input a SIM card at the back of a phone. Just make sure it is in right size and slot, check here for details.
> how would I check to see if the phone is unlocked?
One SIM card plugged in, if you can make calls and use the data with your existing phone from another mobile operator, then it means the phone is unlocked.
> how do I do that? also, I have a samsung 2 that was used with prepaid card.
I need the exact model number of the phone.
> If I wanted to get it to be on unlocked, How much will that cost?
It depends on the mobile operator of your prepaid number. What's the operator's name?