Well your looking at the right places as this is probably one of the best places on the web to ask and get information from as most of us orange encased nutters are signed up for an account here
Just reading these pages will give you loads of good advice and if you still want to participate well....
http://www.bmmc.org.uk/ is where I got started applied gives some good info and questions an being the British Motorsport Marshals Club was for me the best way to get my foot in the door for all the circuits in the UK.
You don't HAVE to join a club but being part of one makes life a lot easier. You get contact information and a structured set of people to help you figure out how to start off and to answer your daft set of first questions ( or at least they did me ).
If your only interested in certain organisations racing then you could become a member of them instead or both to cover all eventualities ( another club that seems to be worth joining is BARC ) but again unless your certain you don't need to go joining loads of clubs.
Once you do join up you are advised to get involved in a Training Day which unfortunately have been organised and run at most circuits already but not all. However don't despair if you are too late as although needed to get an official grading and beneficial to you to understand what the job entails they are available during the year and not having attended one doesn't stop you form actually marshaling events.
You've joined the club, you have a membership number, you feel your ready to try an event, download or visit a volunteering form and fill it in with your details and send it off. You will get a response with all the details of when to attend where to go and what time and then basically you turn up and get stuck in. Tell the guys its your first meet when you get their and they will show you the ropes and hopefully ( given time ) show give you a tour of the posts. You may even find that the chief at the circuit has a spare pair of probans you can use for the day ( thanks for the loan of mine last year
)
You may not get a response until only a week or 2 away from the actual event so don't worry if you don't hear anything back immediately. Oh and don't forget with your membership pack comes those lovely contact numbers for your areas club officers who are brill at setting your mind to rest and pointing in the right direction.
Other than that there are colleges that do courses in Marshalling ( to NVQ2 ) if you wish to go down that route but the qualification is not required its just a different way to get into the job
you still do the same job at the end of the day but you follow a fast track approach to get you trained up with the knowledge to be able to perform as an incident marshal from getting your qual and for me I am much happier to just gradually gain my experience by doing the job and working my way up the ladder as I just enjoy doing the tango trot
so don't need a bit of paper to say I can fall in the kitty litter safely trying to get to my coffee
Its a very necessary job that is performed at every race meeting and without us cant take place, which to me is reward enough for seeing my favorite sport, from the best view in the house, whilst giving some of my time to ensure the participants and spectators can enjoy what I enjoy, safe in the knowledge, that some unnamed, possibly cold, slightly chubby ( some cases ), guy or gal is making sure nothing too dangerous happens on track and is there to give ssistance to participants when it goes ever so slightly wrong.
Good luck with your venture into our madcap world and maybe see you on circuit sometime