Dear Sir/Madam (or a name if you have it)
My name is Tyda. l'm writing to enquire as to whether you might have any volunteering opportunities that would be suitable for me.
I'm really interested in taking on this responsibility because ... ( insert your reason) and think I could be a valuable addition to your team.
I would be really grateful if you could let me know whether this would be a possibility.
Thank you for your time and I look forward to hearing from you.
Kind regards,