There are different things for different people.
For a start, most faculties have their own active society that every student of that faculty is automatically a member of (businesssoc/commsoc, lawsoc, medsoc, etc..). They usually hold regular events like camps and nights out in the city.
Then you've got all the revues. By far, if you want to make friends easily (like guaranteed), join one of the revues (annual skitshow). There is med/CSE/law revue and anyone can join either of these. It's quite a long, tiring experience though but the payoff is the friends you make and putting on the show itself. It's not for everyone however.
There is Yellowshirts but that wont be available to you until later on in the year. On top of that, you have to apply to get in and its like 50% chance you'll succeed or fail (I've both succeeded and failed to make it). It's a similar sort of expierence to revue though. You'll make lots of friends and help out the next batch of idiot first years in 2010.
Then you've got all the other arc-affiliated clubs. Heaps of clubs but I'd say half of them are inactive due to low attendance and a lazy executive. I used to be part of UNsoc and attend annual model UN conferences in NZ and here. Check out the ARC website/ YS website for more info.