You'll need to get into a medical degree. These include the Monash MBBS (which is largely undergraduate-entry, using ATAR + UMAT + interview) or the UoM MD (which is graduate-entry, using GPA (of an undergraduate degree) + GAMSAT + interview). Monash takes 5 years all up, the UoM path takes at least 7 years. After your medical degree, you do your intern year as a doctor (yay!), then apply for the paediatric physician training program, and then move up from there.
In terms of VCE subjects, assuming you'd prefer undergraduate-entry at Monash, you'll need English and Chemistry. With that in mind, it might be advantageous for the GAMSAT (used for graduate-entry) to know physics. The other subjects are up to you; taking three sciences in year 12 is allowed by VCAA. You'd ideally like an ATAR above 99.
If you're set on medicine, I'd advise you apply broadly and look at interstate unis and make sure you've applied for them via their versions of VTAC. Some unis have their own paperwork too (eg. JCU), so make sure you stay on top of all the due dates etc.