When I decided to sell my old iPhone to make a little extra cash, the first thing I did was learn how to completely wipe an iPhone. This is actually easier than most people expect it to be. First you need to make sure that everything is backed up (for example on iCloud). Then you need to go to settings and hunt down “erase all content”. This gets rid of your personal bits and bobs, and will also reset your settings so that the phone is no longer connected to anything like iMessage or FaceTime.