Dear Mr/Mrs. ...

I have seen your job offer posted on the local newspaper and I am extremely interested in this type of voluntary work which is done/promoted by your centre. Fortunately, I have already had past experiecce in taking care of handicapped children as I used to take part in one of the similar activities organized by my school, so I can confidently claim that I am capable of handling this work. Moreover, I find it interesting and eye-opening while doing such voluntary work as I have a chance to devote my time and effort to helping other unfortunate people. I would appreciate the opportunity for explaining how I feel I am suit for this position you are offering. 

I am available for interviews at any time which is convenient to you. You can contact me via email address given. 

I am looking forward to hearing from you.

Yours faithfully,

....

bạn có thể tham khảo bài trên mình viết nhé, còn về mẫu viết đơn từ mình không chắc là có giống với cái bạn được học không, nếu có sai sót gì thì bạn thông cảm nhé.