Focus on 3 things:
1) Academic credentials.
2) Teamwork/Leadership skills.
3) Personal development.
1) There are pre-requisite subjects for intake. You must take these. However, the other subjects that you do at school should support your entry into an airline career. English is a compulsory year 12 subject, so you have to do that. In addition to Maths and Physics, consider subjects like Chemistry, Economics and Geography. The last 2 will broaden your studies away from pure science (giving you writing skills, hopefully) and also explain the sociological/geopolitical/economic background as to why we have aviation in the first place.
2) Get involved in a club or team where you have to engage and work with other people. This could include sailing, gliding, scouts, footy, Air Training Corps etc etc.. Followership skills are just as important as leadership skills and if you can demonstrate participation in club activities all the better.
3) Recruitment involves employing a whole person. Remember that you are a PERSON first and a PILOT maybe 2nd (or 3rd, 4th , 5th...). Develop yourself so as you can learn to accept knockbacks in life and push on ahead. You will only develop this by learning things about your own strengths and weaknesses. Develop yourself by personal challenge: Karate, learning music, gym, bushwalking...I have heard good things about The Duke of Edinburgh Award and Outward Bound-check 'em out.
Good Luck
