[url]http://www.britishcycling.org.uk/web/site/BC/bcf/20081007_membership_home.asp[/url]
Take a look at the link, tells you what you need to get started.
You'll begin your road racing career as a 4th cat. Earn 10 points and you'll move up to 3rd cat (a 4ths win will give you 10 points) Then it gets harder as you try get to 2nd cat like Im planning on doing over the next 2 years. If your really good you may even get to 1st or even elite
I only started road racing last summer, tried a couple at Crystal Palace, struggled from having a broken hand and not being quick enough. Went away, did a bit more hard work on the bike, came back 3 weeks ago and in my 2nd race at Hillingdon got a win.
I'm not to clued up on TT's but there are plenty on here who are so I'll let them tell you more.
Good luck.