Typically, when you purchase a second-hand iPhone, the previous owner will restore the iPhone prior to selling it to you. The restoring process deletes all settings and information from the iPhone and returns it to factory settings. Assuming the previous owner restored the iPhone, you will need to activate the second hand iPhone prior to use.
Turn on your iPhone.
Connect your iPhone to the data cable and connect the other end of the data cable to a USB port on your computer.
Double-click the iTunes icon on your computer to open iTunes. After loading, iTunes will present you with the iPhone activation screen.
Enter your 10-digit cellular telephone number, your billing and the last four digits of your social security number. Click the "Continue" button.
Check mark the "I have read and agree to the iPhone Terms and Conditions" box and click the "Continue" button.
Wait while iTunes syncs with your iPhone and completes the activation process. iTunes will prompt you after your iPhone has been successfully activated. Your iPhone will automatically power off and back on.
If the previous owner did not restore the iPhone, you will not need to activate the iPhone.