Didn't look hard did you - try the "want to get into racing" and "competitions/races" threads in this very forum... 
No, you don't need to be part of a club, but if you want national ranking points you need a licence, available from British Cycling.
It's much preferred by the organisers (and those of us who hate queueing for uplift) if you enter races at least a week (ideally two) in advance, so that the organiser knows how many people to organise transport/toilets/etc for.
15, you'll be in Youth category, unless you want to run in the Hardtail cat at any race that provides one (not all do).

get practising before you race, it doesnt sound like youve had a bighit for long, how long have you been riding?

if im wrong where abouts in the UK are you?

i think there is a midlands series, check out the board on this forum for info, racing is great so id definetly recomend it.