Before you decide to go for it, make sure you have enough training and fitness behind you to manage in a race. It’s essential to have done some regular good quality group riding before you take part in a race. Learning the basics of group riding dynamics such as working in a paceline, also known as through and off, is crucial for breakaways to work if you so happen to get into one. I also went to a few training sessions at the Mountbatten Track in Portsmouth before I had my first race to familiarize myself with the track. If you have the chance to do likewise I highly advise it.
You will need to purchase a British Cycling Membership from the British Cycling website here. Unfortunately cycling will undoubtedly continue to cost you a fortune. To enter road/circuit/track races you will need a Silver or Gold membership. You can choose to get a single day racing licence, however not all races will accept these. More information about these here. If you are a junior you can get membership at a discounted price when you checkout. To find out which age category you fit into look here. If you are part of a British Cycling affiliated team then your license will be sent to your team manager. Once you get hold of your license you need to add a passport sized image to your British Cycling Membership card.
Bad luck if you are a junior or youth rider! If you're a junior rider, to make sure you’re on an ‘equal playing ground’ you get smaller gears than all the over 18s that you’ll inevitably riding against in 4th cat races! Although you probably won't use these higher end gears it does mean you might have to do some fiddling with your bike or buy new parts, as I'll explain shortly...
When your bike is in the biggest gear the distance it takes for one revolution of the cranks is measured and this distance is generally measured in inches. The distance it travels cannot exceed the stated distance according to your age category. To make sure you abide by these gearing restrictions you can either buy new chain rings, cassette and chain or you can use the high limit screw on your derailleur to restrict your gears.
Find a local race on the British Cycling Website under Events and either enter it online (which is generally cheaper and easier) or enter on the day and risk not getting a place. In the beginning you will be a 4th cat rider, which is the lowest category. You can gain points depending on where you place within races to climb up the categories. Find events here.