Researching possible careers is not the sole preserve of a school's Sixth Form so whatever age you are it is never too early to start research.
The internet is full of advice but here are some portals which we are very happy to recommend:
My Career On Line
http://icould.com/
http://www.careersbox.co.uk/
http://www.talkingjobs.net/
https://nationalcareersservice.direct.gov.uk/advice/Pages/default.aspx