My uncle is a 747-400F captain with Cargolux.
You need to get an impressive mark in high school to stand out in the competition.
If you want to get on with your career straight away, you apply for Cadetship, Qantas, Etihad, CX, etc. It will take a lot of time and patience to get to the seniority of becoming a 747 pilot. Until then, you will start off with turboprops, then advance to medium-bodies.
If you want to try air-force, you can but you have to serve with them for around 11.5years(i think). Even after that, you won't be guaranteed a 747 position, but you will be recognized more.