Where or what clubs on campus are best for meeting people? What volunteering positions do people recommend?
I think this is a fairly subjective issue. You'll probably gain more enjoyment from the clubs that are actually aligned with your personal interests. You can explore all of these options during O-Week
I'm personally not aware of too many volunteering positions that run at Unimelb. One I can recommend, however, is Open Day! IIRC applications come out in June, and you can volunteer to be a tour guide, stall member, etc. It's a really great day, and you meet some wonderful people. However, if you are looking for a more sustained volunteering role, it might be worth waiting for the response of another user or asking around during O-Week.