If you still have your iOS device

Before you sell or give away your iOS device, make sure that you've removed all of your personal information. Follow these steps to protect your data and get your device to its factory default state for the new owner:

Back up your device.
Go to Settings > General > Reset, then tap Erase All Content and Settings.
This will completely erase your device and turn off iCloud, iMessage, FaceTime, Game Center, and other services.
If you're using iOS 7 and have Find My iPhone turned on, your Apple ID and password will be required. After you provide your password, the device will be erased and removed from your account so that the next owner can activate it.

Contact your carrier for guidance on transferring service to the new owner.

When the device is turned on for the first time by the new owner, Setup Assistant will guide them through the setup process.
Important: Do not manually delete contacts, calendars, reminders, documents, photo streams, or any other iCloud data while signed in to your iCloud account, or the content will also be deleted from the iCloud servers and all of your iCloud devices.
If you no longer have your iOS device

If you didn't follow the steps above before selling or giving away your iOS device:

Ask the new owner to erase all content and settings as described above.
If you're using iCloud and Find My iPhone on the device, you can erase the device remotely and remove it from your account by signing in to icloud.com/find, selecting the device, and clicking Erase. When the device has been erased, click Remove from Account.
If you're unable to complete either of the above steps, you should change your Apple ID password. Changing your password won't remove any personal information that is cached on the device, but it will make sure that the new owner can't delete your information from iCloud.