[deley]

Generally speaking volunteering is for a Club rather than a circuit (e.g. BRSCC, BARC, MSVR, etc. rather than Brands Hatch, Croft, etc.)
Volunteer at any time of year but most marshals volunteer in the early part of the year as the various Clubs publish their schedules.
As you say that you are new, are you looking to marshal at meetings this year (there are still some to go) if so contact the Club organising the meeting - you can find the chief marshal or marshal contact on most Club websites.
Which circuit would be your "local" - someone will be able to add any additional details.
Dave