Most Australian states require subject selection in Year 11 (around March/April). There's usually a deadline and a process: form to the school, confirmation with parents, sometimes an interview with a guidance counsellor.
Start conversations by Year 10 at latest. Year 11 is thinking time, not "we decide at the deadline" time.
The process varies by state. NSW has the HSC, Victoria has VCE, each state has its own. Ask your school for the exact timeline and requirements.