You can get lots of good advice from the
Institute of Careers Guidance links page. This provides links to the Careers Counselling Services of Scotland, England, Ireland and Wales, as well as a few other useful links.
Jobcentre Plus has a very useful page too, with links to education and training resources, as well as jobseeking advice.
AimHigher also has some good careers advice on its site. There are great pages for 'comparing' careers and a very useful career profiles section too. If you're still not quite sure what career is right for you, this might help you work out what it is you want to do.