Honestly, it really depends which scholarship you are applying for. Many times there are requirements on what you need to include in the letter, and this is heavily dependant on the actual scholarship. Obviously applying to a corporation scholarship from Pepsi, will be a bit different than the requirements for a academic essay type scholarship.
After a quick google search I found these articles somewhat helpful - but you need to be sure to check the actual scholarship guidelines of the committee your actually writing for.
Ehow has some good guildelines in this article - here
Another comprehensive list: writing loversscholarships.com
is also a good source - they have a lot of scholarships and details on how and when to apply.scholarshipsngrants.com
has a list of scholarships if your just getting into college, as well as a few tips and resources on how and where to apply.