When you first leave school you may be so keen to earn money that you will take any job going. This can be fine for a while but eventually you may decide you want to do something more satisfying, that has long term goals and prospects that you can aim for. This is when it's time to start thinking seriously about choosing a career.
The best starting point is to think about your personal interests and strengths and what sort of personality you have. Are you outgoing, shy, caring, technically minded? Try and find a career that builds on your strengths. Some careers will require futher study for professional exams, while others may require you to be self-motivated working as your own boss.
A good starting point is your local careers centre, this may be within the school or college. These can supply you with employment personality questionnaires, which may give you a good idea of what would suit you.
It may also help to do voluntary work experience in a variety of work settings so that you have a better understanding of what the work will involve and whether it would suit you. This sort of experience will also be a useful addition to your C.V.