My question is how to write a motivation letter for volunteer and I am a stay at home?

my question is how to write I am a mother of three and out of job since 1996 . I have to Bachelor of arts degree from two different Universities. the recent was 2008. IAs mother of three children I was focusing on them and now I have decided to join and do something both for myself and the community so can anyone suggest how to write motivation letter for United Nations Volunteering