Most high schools will hold an information session about university applications, and in my experience most also have on offer free UAC (if you are in NSW) guides which have basic course details, entrance requirements and application procedures. Also, in about August-ish graduating students are usually sent a little slip with their "log in" details for UAC so that they can apply for university if they so choose. Beyond this, its totally up to you. Also, they usually only give informations for the university application process for the state that you are in. Each state has its own application system with different fees and deadlines. All applications are able to be completed online.
If you are in NSW the applications open August I believe. Then you go to the UAC website, put in your received log in details, fill out your details, pay the fee then add the preferences for the courses you want to apply to. I believe applications generally close in September. For Victorian universities you use VTAC, for Queensland universities you use QTAC, SA it SATAC, Tassie and NT I believe you apply direct to the unis, and WA has one that I have forgotten. Each of these is accessible online. It gives instructions as you go along.
You will only generally receive one offer, but as the state application places are separate you can receive one offer from each.
Each course has an ATAR entrance which is determined by the demand of that particular course. More popular courses will have higher ATARs. Often people do not get the ATAR they need to get into a course so they start another course with the aim to raise their ATAR. This is much more do-able at the lower end of the ATAR scale.
For example if you had an ATAR of 65 and needed one of 70 to get in you would do one years study, and just sitting on a pass average would give you an GPA selection rank of about 79 (or something like that). When averaged out with your ATAR this would make your selection rank 72. Sitting on a credit (which I believe is achievable for all university students) would raise your GPA selection rank to about 88 which again lifts your marks up more, but this number is always averaged with your ATAR. Unfortunately this can also work the other way around. If you had an ATAR of 96 but only achieved a distinction average (about 94) then theoretically your rank would be pulled down to 95.
As you can see if you were aiming for a much higher ATAR for something like medicine or law it may prove very difficult to get in because you have to consistently do well and already have a very reasonable ATAR.
I hope that made sense.
