The below should work. Most of the triggers are self explanatory. So for example if you need something after a user registers then onAfterUserRegistration is typically the trigger you'd use. You can also use onAfterSaveUserRegistration if you want to execute something at the absolute end of frontend registration. It's fired after user is saved, emails sent, etc.. are all done and is just before the registration complete message, but for most behavior that modifies the user onAfterUserRegistration is recommended.
Global
Triggers: onAfterUserRegistration
Type: AcyMailing
User: Automatic
Access: Everybody
Conditions
1: [FIELD_NAME] Is Equal To apprentice
As for what variables are available to a trigger you can find those in the below documentation.
www.joomlapolis.com/documentation/279-community-builder/tutorials/18358-using-cb-triggers