How do i Write a letter to get sponsors?

Well, recently I received a letter to participate in the "Memphis pageant for teens." Today I found out that I made it to the preliminaries, and the top the winners from the preliminaries will get to go next year to nationals. My problem is that i need sponsors, I need a total of $620.00 ($495.00 for preliminaries cost and $125.00 for photogenic contest). My biggest problem is that everything has happen in such a short time and I need $275.50 by April 11th (11 days from now), this is NOT only a pageant but is also a scholarship program. I really need to write a sponsorship letter. So here is where you all come in... How can i write the appropriate letter???