Rather than butcher other people's copy on the subject (as most of the internet seems to) I'm just going to refer you to an excellent beginners guide to sports trading that was written by The Gambling Times. It covers all the basics of what, how, why and when for trading.
It's not the only information you'll need to get started, but it's definitely a solid start. Spend some more time reading through their gambling guides and you'll start to build up a knowledge that will hold you in good stead, and reduce potentially costly mistakes.