Unbox your iPhone and turn it on by pressing the On/Off switch. A welcome screen greets you, displaying a Slide To Set Up slider that rotates between different languages. (If you need quick access to your device's IMEI or ICCID number without setting up the phone, you can tap the information button [represented by a lowercase i] located directly above the slider.)
Once you begin the activation process, you're asked to pick your language and country, and whether you'd like to enable Location Services. This allows Apple apps (and third-party apps) to access your location via Wi-Fi networks and your GPS (Global Positioning System) location. Your iPhone then checks for any Wi-Fi networks in the area that your phone can connect to; if there aren't any, or if you'd rather use your cellular service, just tap the Next button.
If you're activating carrier service--which, unless you managed to pick up an unlocked phone, you most likely are--you'll see a message asking you to confirm the phone number you're activating this iPhone 5 with. In addition, you'll need to supply your billing zip code and the last four digits of your social security number. Apple's servers will then communicate with AT&T to activate your device.
From here, you can set up your device as a brand-new phone or, if you're upgrading from an old iPhone, you can restore your data from an iCloud or iTunes backup.
Restore from an iCloud backup
If you have an iCloud account and have backed up a previous iPhone or iPod touch incarnation using iCloud's Backup feature, you can use this backup to restore your data to your new iPhone (though you'll need to be on a Wi-Fi network to do so). To restore your data, sign in to your iCloud account; agree to Apple's terms and conditions; and then choose which backup file you'd like to use, and tap the blue Restore button in the top right corner of the screen. (Depending on the speed of your Wi-Fi connection, this process can take anywhere from a few minutes to a few hours.)
Restore from an iTunes backup
If you tap Restore From iTunes Backup, you're brought to the Connect To iTunes screen. Connect your iPhone to your computer and open iTunes; after clicking your device in the source list, you'll see the Set Up Your iPhone screen, which asks if you'd like to set it up as a new iPhone or restore from a specific backup. Choose the correct backup, and click the Continue button to proceed. This process is significantly faster than restoring from iCloud, because you're transferring data over USB, not over Wi-Fi.
Add an Apple ID