UAC applications open in August and close on the 30th of August ...i think (just google uac and it's on their home page)
basically you pick 9 courses and order them in preference of which one you'd rather do - then after HSC you receive an ATAR and main round offers for a uni place happen on the 20th of January. This is when you find out if you got a high enough ATAR to get into your first preference, if not you may be offered you're second preference etc.
All up there are about 3 or maybe four rounds - an early round, a main round and a late round - if you're lucky you may get an offer in all rounds, but you can only receieve a maximum of one offer per round.
The UAC book comes out in August too and that's when most uni's hold their open days.