You should have a clear idea of the type of organisation you wish to work for and the type of role you are interested in observing. (careers.manchester.ac.uk)
NOTE: If you are not sure which keywords to use, look at several job descriptions for roles you are interested in and qualified for; see which keywords potential employers are using. (aspireforsuccess.com)
Whatever your reason, a strong cover letter will help you to gain positive attention and — hopefully — to garner a personal interview for the volunteer role you are interested in. (thebalance.com)