First off, do you already have a racing licence? No big deal if not, it’ll just cost you afew quid more on the day (you’ll need to buy a Day Licence in addition to the entry fee).
Have a look on British Cycling, click on events then select your area, narrow it down to road races and see what there is. You’ll be restricted to Regional level events (Regional A = 2/3/4, Regional B = 3/4 and regional C+ = E/1/2/3/4 or sometimes 4th only). The number refers to the category of racer and as a newcomer to the scene, you’ll be a 4th Cat when you buy a licence. You should be able to get an idea of how difficult the race will be by looking at the categories it’s open to and the distance, the map link on the webpage may also give you an idea of how hilly it’ll be.
Choose your race and enter – some you can do online, others you’ll have to download a paper entry and send it off with a cheque.
Turn up on the day, ride as fast as possible.
Easy! 😉
Have a look here:
http://www.britishcycling.org.uk/membership
for more stuff about racing licences, BC membership etc.