The Short Answer
There's no single subject that's "right" for everyone after matric, but there is a right *process*. Start by understanding your strengths, interests and personality (an aptitude test helps here). Then explore three or four careers that fit that profile, check which qualifications they need, and compare institutions on cost, location and accreditation before you apply. Your APS gets you into a course. It doesn't decide your future. If you're still unsure, choose a broad, flexible qualification (like a BCom, BSc or diploma) over a narrow one, and give yourself permission to adjust course later - most people do.