If you are seeking to give some of your time to be a Volunteer then they will be looking at what skills you can offer them in relation to what they are looking for and have a gap in their establishment.
To help you with a letter for your application please see below:
Cover Letter: Volunteer Job
Application for Volunteer post
I would like to submit my CV for a Volunteer post with yourselves. I have always been keen to start some volunteering work and feel that I have enough experience that you would be interested in.
(Tell them a bit about what you have been doing) in previous employment, work experience, courses, training etc.
I am an energetic individual who enjoys helping out wherever I can. (Have you done anything else similar which you can use here?)
I am computer literate with experience in Microsoft Office packages and a quick typing speed. I am organised and thorough and able to deal with paperwork effectively. (Depending on what your experiences is will depend on how much you can put here, e.g.; do you have any finance experience? Could you help them with the books or invoices etc? Do you have any catering experience so could help with drinks, lunches? This all depends on the organisation that you are hoping to volunteer for and what your previous experience is).
I have a clean driving licence and access to a car so can be mobile during the day as needed. (Only if this is true)
Should you be interested in the skills that I have and think that I could help be a volunteer with your organisation then please do not hesitate to contact me and I will be happy to answer any questions you may have.
I look forward to hearing from you.
Good luck with your application.