Be attentive to detail. The employer can evaluate you easily as a potential employee based on your writing ability and professionalism. Check and double check for misspellings, typos, erasures and grammatical errors. The first paragraph should state why you are writing. If you are inquiring about potential opportunities, describe the type of position you are seeking.
Specific statements are far more effective than vague generalities.
The second paragraph should state the reason you are interested in the organization. Show from your research why you are impressed with the employer. The next paragraph should elaborate on your background and your qualifications for the position. Expand on important resume entries. Mention any pertinent facts which do not appear on your resume and are relevant to the situation.