Dear headmaster（你没说清楚那我就默认这封信是写给校领导的）
My name is XXX. I have heard about the recent activity to the old people's house which I have great intrerest in and I would like to be a volunteer. I am a person with a warm heart and I like to help other people. I am aware that some of the people in the old people's house may not have the ability to move freely. I am very happy to take them on a walk in the garden on wheelchairs. I can read the paper for people who can't see very well. I can also help and clean their rooms. I am always free during weekends. I think this would be great oppotunity and important expereince for me and might be useful in the future. Please contact me.
Regards
XXX
我现写的,120字左右,没有什么难的时态或者词汇,主要讲了你乐于助人,可以去帮老人整理房间,散步,读报纸.周末有空.认为这是一次机会,能够成为宝贵的经验或许对将来有用.
望采纳