You can't volunteer during the close season as fixtures will not be made public until some time in the new year, for most circuits towards the end of January or early February.
You should be looking at doing a training day at a circuit convenient to you , which will be held early next year. There are still a few race meetings left this year - it may be worth trying to do one as training will make more sense if you've already done a day or two.
Most marshals are members of the BMMC. Take a look here:
https://www.marshals.co.uk/
for more information.
It would help us give you more specific information if you told us what area of the country you live in.