If you already have a natB license youre halfway there mate!
First join the BARC as a Yorkshire centre member http://www.barc.net/regional_centres/index.php
(it can be a struggle, websites not the best!)

Then enter an event here http://www.harewoodhill.com/
Got to be a good starting point being fairly local?

There is only one event left and its this weekend, and entries are closed so youve missed out this year.
A good start would be to enter the pre season practice day next March, keep an eye on the website. Also plenty of info on the website about the class structure- basically the extent of your modifications will dictate the class you run in... There is no class structure on practice day and you could also get advice on what class to enter on competition days.
Hope thats helpful