Not sure where to start with this one, it's a little messy.
Where are you located? (this matters for; car, home purchase). You might need a 4WD if you need to travel in the outback. IF you live in Sydney, you might not be able to afford to buy a home in the next couple of years while you put together a deposit.
What's your industry? (this matters for; car, home purchase, mortgage). There might be tax implications if you travel a lot for work. Depending on whether you might be moving in the future due to your industry, think long and hard about buying a house. If you will be working from home for significant amounts of time, there might be mortgage implications.
Figure out your "needs" vs "wants". In your situation, without telling us your savings, a $15,000 car is not a need, it's a want. You can get a good, reliable car cheaper which won't see you in significant debt. Don't borrow from your "inlaws". My example for the car situation is this: I graduated, had savings, bought a reliable car ~7 years old with 50,000k's on it for $6000. It's extremely reliable, looks awesome, cheap to run. I didn't get a loan. I still have the car, 4 years on. It's taken me from Adelaide to Canberra and Adelaide to Sydney and back many times.
You haven't listed your savings. If you want to buy a home to live in, it's always good to have at least a 20% deposit saved.
Helping out your parents is not tax deductible. Not sure why you would be doing it on your salary. If you're talking about helping out as "board" as in "I live at home and want to pay for myself because I earn a wage", that is not "helping out your parents", that is more along the lines of being an adult. Talk to them, figure out an amount that is acceptable. Think about bills, food, space etc.
What's your relationship with your girlfriend? Is she employed? Do you see a future together?
Student debt is a matter of HELP, when you fill out your tax form at your job you need to tick that you have a HELP debt and repayments will be withheld from your salary accordingly. You can volunteer to make extra repayments which currently attract a small discount. The debt gets adjusted according to CPI each year.
Sounds like you've got a lot of reading to do :)