How can I write a letter asking for donations?

Today I received very bad news, I was informed a few days ago by my GP that I have a life threatening medical condition and the GP informed me that I need to operated within the next six months or else my medical problem could result in complications that will end my life… The bad news is that I don’t have medical aid and I am only a contractor. I went to a government clinic only to be told that I will be placed on a waiting list and may only be operated on in a few months from now. That may be too late by than. My GP referred me to anther specialist to see if he can help me with my problem and his diagnosis was the same. A friend suggested that I should start a fund raiser by circulating a donation letter and asking for donations. I figured I could e-mail and post a letter but I honestly don’t know how to write a letter for asking donations. Is there any one who could help me to write a letter or have tips on writing a donation letter?