Before you sell or give away your iPhone, be sure to erase all content and your personal information. If you've enabled Find My iPhone (see Find My iPhone on page 38), Activation Lock is on. You need to turn off Activation Lock before the new owner can activate iPhone under his or her own account.

Erase iPhone and remove Activation Lock. Go to Settings > General > Reset > Erase All Content and Settings. See support.apple.com/kb/HT5661.