First you have to look at what areas you might be interested in. Do you always wanted to work with animals? Do you like reading a lot or are you fascinated by cars? Write down these areas of interest. You are going to be better choosing a career in something you naturally feel close to.
Next you need to know about your abilities. This means not only your pure exam results and qualifications, but also your aptitude. You can also discover your perfect careers & jobs by uncovering your work personality strengths via online career tests.
Take some aptitude tests to show you what you might actually be able to do. You may like cars but have no spatial ability. Engineering may be out but design might be a possibility. You may like books, but should you be writing them, publishing them or selling them?
These tests will help clarify your character and inherent abilities and point you to areas to which you are most suited. Find out as much as you can about careers in the area you have chosen. If you want to design cars, what sorts of qualifications are needed?
Could you ever get them? Do people usually learn on the job or are exam qualifications essential? Is this a growth industry or a fading art? Roof thatching may be fascinating, but is there a future in it as a career, rather than a hobby?
Go to careers fares and see advisers. Libraries and the internet will help with research. Go to the major players in the areas you are interested in.