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.
You’ll also want to know how to make that FaceTime call once it is activated on your device. There are more instructions here:
Apple dropped a bombshell at WWDC this year. A long-desired feature to go beyond a two person conversation to include up to thirty-two individuals. With iOS 12, Apple has changed the game significantly. Seamlessly move from iMessage to Facetime in audio or video with your closest friends and family.