FACETIME- how to set to auto turn on ?

Add it to your Login Items in System Preferences > Accounts / User Groups.

To learn more about Mac OS X: Helpful Information for Any Mac User by GGJstudios​

 

OP: Is this what you mean, or do you want your computer to listen for incoming calls when your computer boots up?

 

Then you shouldn't have to do anything. There's a process called "imagent" that starts up automatically, and if there's an incoming call, it will launch the FaceTime app and ring. You don't need to have the FaceTime app open to get incoming calls.

jW