It depends whether or not you're rural or local so first things first you need to figure that out. Basically local is sydney, blue mountains and newcastle. But just google "rrma ratings" type in your town and if its 4-7 you're rural, 1-3 you're local.
Once you've worked that out you need to organise to sit the UMAT in about July but they won't release the date for a bit yet so just keep it in mind. It'll be near the beginning of term 3. You might also like to enroll in a UMAT prep course but it's not necessary, just something to consider. Everyone needs to sit the UMAT so DONT FORGET!!!
You will also need to fill out an application form which you can download from the unsw med site, there is a different one for rural and local. Then fill it out and send it back to them. You also need to organise for your principal or year advisor to complete an academic reference and ATAR prediction for you. Again this can't be done until you've done a majority of your assessments.
If you have a combined UMAT and ATAR prediction which meets the cutoff they will invite you to an interview. Rural interviews are in September and I think local are in late november (can someone verify this).
After all that's complete, it's out of your hands and the hardest part of all starts: waiting!
As far as requirements go, for rural you need an ATAR of at least 90 and a UMAT of at least 135 however people with such (relatively) low scores will have completely hammered the other sections. Ie. Someone gets an ATAR of 92 will need to have had an amazing interview and a very high UMAT to be offered a place.
Local needs an ATAR of 95 however I'm not sure what the UMAT requirement is but it's much higher. Local is incredibly more competitive than rural.
None of this needs to be started for a few months yet though so for the time being work your butt off to get the ATAR you need.
Hope this helps
