Open the FaceTime app and sign in with your Apple ID. You can also do this from Settings > FaceTime.

If you're using an iPhone, FaceTime automatically registers your phone number. To also register your email address on your iPhone, go to Settings > FaceTime > Use your Apple ID for FaceTime, and sign in with your Apple ID.

If you're using an iPad or iPod touch, register your email address by tapping Settings > FaceTime > Use your Apple ID for FaceTime, and sign in with your Apple ID.