What sort of events are you looking at? Road or off-road? Most off road mountain bike events have a "Fun" category where you do a couple of laps of the course all the more experienced racers do. If it's road you're after then most of the entry level events will be on closed road circuits away from traffic - look for Go-Race events on our calendar. If you want to compete on the open road, then sportives or reliability trial events which most good cycling clubs hold would be worth a look at - whilst not technically allowed to be called a race, they allow you to test yourself against the clock and there will usually be a good group to ride with without the pressure of racing. There's plenty  of options out there for you!