An unlocked iPhone can be used with SIM card of any Service provider. Of late unlocking iPhones has become very easy and all you need in a PC to do it. There are many unlocking software available free of cost, which unlock the iPhone at click of a button. ZiPhone is a very popular software for unlocking the iPhone.
If you are not sure of doing it yourself then you can always get it done by a professional. Contact Dandy. (Search his posts in QL for his contact number.)