Hi ash17 - Have a look at this website, it has quite a lot of information on it.
http://www.citb-constructionskills.co.uk/cards chemes/
Basically to qualify for a CSCS card you need to be able to pass the Health and Safety Touch Screen Test. This is a very basic test that proves that you have some knowledge of health and safety on a construction site. Once you have passed this test then you will qualify for a CSCS card.
The test itself costs �17.50 and a CSCS card costs �25.
As things are quite tough in the Construction Industry at the moment a lot of employers will not employ you unless you already have a CSCS card.