Your passion is great - if it doesn't go away you should have a successful career as a pilot.
Should I start flying now? Should I get a private pilot license by 16?
You certainly can, but there's no rush. It might be worthwhile focusing on your schooling first, because if you don't do well in that you won't become a pilot.
What should I study for VCE? Do I need to go into aviation for university, or is there a special flying university?
Maths and Physics are the big subjects and you'll need to study them through to year 12. English is also an important one. Other than that study subjects that interest you - you're more likely to score well on those. You do not need to go to university to be a pilot but it can be an advantage. Some universities offer a Bachelor of Aviation (usually management stream OR flying stream). But if they aren't an option for you any science degree will look good on your future CV. The flipside is that you'll be paying more than someone who just joins a private flying school, of which there are heaps in Melbourne, and you'll be a bit older when you start your career in earnest.
How bad is the competition?
You'll hear many people saying there's a pilot shortage, and others saying that is a myth. The truth is somewhere in the middle. There are lots of pilots who have recently come out of flight school who can't find their first flying job. That is undoubtedly the toughest step in a piloting career. But once you are more experienced and have specialized qualifications (for example, large seaplanes), you will be highly sought-after. Airlines will always have more applicants than jobs but in the last few years it has been really good for pilot hirings, especially compared with 5-10 years ago. What happens in the next few years though is anyones guess.
What are all the stages of becoming a pilot starting from 15 years of age?
The first step is to contact a flying school and ask for a trial flight. After that you decide if you want to continue, and one of the first things you should do is get a medical clearance. You then complete your recreational licence, your private licence (optional but recommended), then your commercial licence, with a whole bunch of theory exams throughout that. Following that you should get your instrument rating or instructor rating so you can get a decent job flying light aircraft. Do that for a few years and you'll have enough experience to apply for the airlines.
However there is a new route that is growing - airline cadetships.
Should i enrol in a flying school now?
This is a tough one to answer because there are pros and cons. My personal advice would be to wait. Qantas just announced that they are opening a large training organisation. Details are scarce but it will probably operate replace their airline cadetship scheme - meaning you train with them and get a job with them straight away, instead of flying light aircraft for a few years. If it's anything like their schemes in the past, they won't want people who have already begun flight training. You will need to have graduated high school or university before joining them, so I'd make that your first priority. If you miss out you can then enrol in another flight school or university, do an 18 month full time course and come out the other end as a pilot.