It can sometimes be difficult to know what to write in a cover letter. You want to convey how excited you are about a position and how qualified you are for it, but you don’t want the employer to misinterpret it into thinking you are arrogant. The following article states that with cover letters, it is best to keep them simple and effective. They then go on to list items that should never be used when creating a cover letter: http://career-services.monster.com/yahooarticle/worst-things-to-write-in-cover-letter#WT.mc_n=yta_fpt_article_worst_things_cover_le.
The library is also a great resource to use if you need guidance on writing cover letters. There are numerous books on the topic that are available for checkout!