Your
Connexions adviser will help. They may be able to help you make the decision by talking you through weighing up your options.
Teachers can help you work out what you're strengths and weaknesses are. They may be able to help with work experience.
Careers teachers particularly, have experience of helping people find out more about careers.
Parents and Carers can help, they know you well and can talk through with you to help identify your skills and abilities.
Friends and relations may know about careers you don't know about and they might be able to help you work out what you want out of life.
What if you get it wrong?
If you do enough research and think hard enough about your wishes, ideas and options this may be unlikely to happen, however it is extremely common for people to change career more than once in their working life.
There is help and support available for people in such situations.