If you have a phone that uses code division multiple access technology to transmit information, contact a cellphone carrier that supports your phone to activate it. The carrier may need to reprogram your phone in order for it to work on the carrier's network. Provide the phone's 18-digit serial number to the carrier so it can register your phone in its system.
If your phone uses the Global System for Mobile Communications to transmit information, the phone should have a slot for a subscriber identity module card. If you don't already have a SIM card, you can buy one from a cellphone carrier. If you purchased the cellphone from a particular carrier, you may need to use this carrier's SIM card. If you have an unlocked cellphone, you can use a SIM card of any carrier.
Once you have your SIM card, insert it into the SIM card slot of your cellphone. If the SIM card you have is a pre-activated one, then wait for the name of the carrier to appear on your cellphone's screen before you use it. Otherwise, call the customer service hotline of your cellphone carrier, and have a representative activate the SIM card for you.