An unlocked iPhone means that it is available to be used on any network. These are typically the most expensive versions to buy since many carriers discount iPhones in order to encourage you to sign up for a contract. However, for an unlocked iPhone you can choose any carrier and not have to be tied into a contract. In fact, if you have separate SIM cards for a variety of carriers, you can choose more than one in order to use your iPhone if you choose to.
Unlocked iPhones do not come with a SIM card, so you do have to supply your own. You want to make sure you get the correct one. For example, the iPhone 4 uses a microSIM card and other types of SIM cards won’t work. Also, make sure that the cell phone carrier and SIM card you will be purchasing from them will support your iPhone version. While most of the larger carriers such as Verizon and AT&T will work with just about any of them, the smaller carriers don’t always provide services for all iPhones.
Once you have a compatible SIM card for your iPhone you can activate it by inserting it into your iPhone. Then connect your iPhone to iTunes to complete the activation process.