Student number/Uni ID is on your enrolment form which you'll receive in the post in the next few days. Enrolment for all students (except straight BA) is in person at Melville Hall, on the dates listed in your enrolment package. If you're in a residence they'll probably organise a group trip down there on the Halls and Colleges enrolment day.
Not sure if it was different in previous years but last year, they made you type it in yourself on the Chifley Library computers after they activated your account.
Oh, and ANU prints the dodgiest student IDs ever. :/