I need help writing a letter please easy 10 points :) ?

I'm going on a student trip that i need to raise around 6500 dollars for, and my mom suggested trying to get companies to sponsor me, like my grandfathers company (he owns a large lumber company and a large real estate company) and my mom's office (they sell educational products) to try to get them to maybe donate some money or something, and she suggested to write a letter to them. How could I word that? How would I start out? I want to be polite as possible and I know in the introductory paragraph I'll explain what I'm doing and what I need but beyond that I'm lost.