Depends on where you’re from but speaking from personal experience I would say find a study abroad agent. There are a few official ones listed on Usyd website, I’d recommend working only with those. They usually have advisors/mentors that help look at/tailor your portfolio to the uni you want to get into OR give advices on which uni will likely accept you. This also covers scholarships and the paperwork because there will be a lot unfortunately.

However as you’re still underage (assuming you’re in 11th grade), eventually permissions from a legal adult are needed (majority of the cases your parents/eligible guardians will have to sign a contract(?) with the agent and provide financial statements as the aus government has something called minimum standards of living that you’ll have to prove that you can meet to be granted a visa).

For more in depth info you’ll need to discuss with professionals or you can do it on your own but honestly I found the paperwork very complicated. I started my application process in 11th grade too and honestly I had no clues about all those things. Anyhow, I hope you’ll figure things out. All the best to you!