Join a motorsport club local to you. You can get involved in helping out in all sorts of ways from organising, marshaling, helping other team / club members etc then the natural progression is to build up your own team and get out there competing.

Trackdays are a good safe way of learning some of the skills required, so if you have a road car then get it booked on to Donington and go and have a play. If you need help finding a local club near to you just post up for more help