One possible version
Dear Headmaster,
My name is LiHua.I hope to be a volunteer in the new library.There are some reasons why I can be a good volunteer.First of all,I don't have any plans during my winter vacation.I have enough time to do it.And also,I'm very outgoing,I'm good at English.Besides,I know much about our library,I can communicate with other guests fluently.I think it's my duty to provide other students with excellent service.It's really a good chance for me.I can also practice my oral English through this job.
Hope to be one of the volunteers.Looking forward to your reply.
Yours,LiHua
