Stay at Aussie Way Backpackers.
Small hostel, nice pool, maybe not the cleanest, but it was perfectly fine, and I had a brilliant 3-4 months staying there.
Only about a 10-15min walk to the city, and right by Roma St station where you'll get the train to from the airport.
House shares, you're better off waiting til you have a job and you know where you're gonna wanna live. Also, I had waaay more fun in a hostel than I did in the house share. The people there weren't really the most sociable, and when you're at a hostel there's always something going on, which is a positive.... but can also be a negative, BUT at aussie way they had a curfue at about 1030, so you could always get a decent night sleep. And only 4 in a dorm, if you ended up staying a while there you end up with the same people in your room, so you can trust them etc....