Be professional, cordial and informative. That means to use proper grammar and letter format, to not be overly aggressive and to state your qualifications for your job you are seeking and your desire for employment with that company for several reasons such as being your current and future career plans, the reputation of the agency for doing quality work any any other positive thing you can add that shows you know what the company does and how you can fit into the organization. Do not raise salary or benefits issues at this point. That can be discussed at the interview, if you get that far. The need of additional employees vs. the number of qualified candidates that may be available, will have an impact on your success.
If possible, mail the letter. If the company insists on an e-mail submission, write the letter in Word and attach it as a Word Document or PDF. You may also paste the text of the letter in the e-mail, with a notation that a printable copy is attached.
Keep it to one page. Do not write excessively long paragraphs. Double space between paragraphs. Usually it is also advisable to attach your resume to the letter, either as a second page to the snail mail version or as a separate attachment to the e-mail.