If you want to set a fields value use the Field action. As for the trigger use all of the following if you want it to apply to frontend and backend registrations as well as profile edits.
onAfterUserRegistration, onAfterNewUser, onAfterUserUpdate, onAfterUpdateUser
Below is a breakdown of where these triggers apply.
onAfterUserRegistration = frontend registration
onAfterNewUser = backend registration
onAfterUserUpdate = frontend edit
onAfterUpdateUser = backend edit