For me personally, it's figuring out what broad areas I'm interested in (e.g. physics and maths), looking at majority of jobs in that field and cutting out ones that don't interest me (e.g. physicist, engineer), then looking at every possible course for those careers and noting the entrance requirements (spesh, methods, English etc.).
From this, I concluded English lit, methods, spesh and physics, which left me with two spaces left. From there I picked a subject which opens up door in case I change my mind (chem) and a completely different subject which I find interesting (business). Obviously, these results will differ from person to person, but that's the way I chose subjects and what I'd recommend to anyone.
In my process, the only role a career counselor really played for me was showing me which subjects my school offers and linking me to some courses based on my interests. I wish I had (and have, as I'm only in year 10), more guidance from the career counselor. Having been in two schools and seeing 3 careers counselors, I found they mostly just have things which would take you 5 or 10 minutes to find on the internet on demand (e.g. course info). I also wish there was more opportunities to talk to people from related fields, and a more strong work experience program (personally, a week in one job isn't enough, it should be a 2 week course, 2 or 3 days in 3-5 different jobs of interest, and this would create more use for a career counsellor helping kids find those interests and jobs to suit those, as normally just 1 work experience placement can be difficult to get).
I hope this has helped!