It has been a few years since the last time I looked at the thresholds for entry into undergrad medicine; others can probably rattle off which universities have the most favourable admission requirements -- and yes, they do differ by university -- as well as the best UCAT resources.
It is fair to say though that entry into medicine is extremely competitive, and that you need both a great ATAR and UCAT result to be successful. For instance, I know several people with an ATAR of 97+ who didn't get into Monash med because their UCAT results weren't great, but rather 'only' very good. This means that most people do prepare for the UCAT, usually starting as early as early in Year 12 (and in some cases, even earlier).
Getting a high ATAR score is a function of a lot of things, and obviously there's no single answer (most helpful advice is probably subject-specific!). Having said that, two things I have found helpful:
(1) Use the resources that are available to you. You can cast a really wide net. You have your teachers, your peers, Internet videos, this forum, other textbooks, and so forth. If you don't understand something, the worst thing you can do is completely give up on it. (You might be able to use them right now to clear up some of your difficulties in methods, or get a head start on English.)
(2) VCE subjects are based on a finite subject design, and usually have a finite pool of questions (or types of questions) that they can draw on. For instance, in English, text response essay questions usually touch on one of a few themes. In Methods, there are only so many concepts you need to know, and different ways to ask fairly similar things. This means that once you understand the content (e.g. the text you are studying, or the topic in maths) and have prepared for the types of questions they are likely to ask you about it, you are usually set to answer what they'll throw at you in the SAC/exam! Having trouble with understanding something, answering a particular question, or finding questions to answer? See (1).
I am a firm believer in keeping doors open at every step along the way at this stage of your life. You sound quite motivated and also mature in wanting to be realistic with your expectations. The only downside I can see to going for it is that you will have to balance UCAT preparation with your VCE studies (and others will probably have more specific tips on this, again). But an upside is that you will very likely push yourself towards getting a higher ATAR, meaning you will have other options open to you, and many of them may have the potential for graduate medicine entry.
With regards to those other options, it is very worthwhile to research them, especially with regards to how they lead to medicine, what other career options are available, and your interest in that degree. Graduate entry into medicine is also very competitive, although far from impossible (and in many ways, university is quite different to high school, and serves as an equalisier and a fresh start). And as I said before, it is really important at every stage along the way to consider other options, as well as your personal interest! There is no point going to university if you are going to hate your studies.
Hope this is helpful - it's pretty late and even my best editing might mean this is an incomprehensible rant. Good luck!