When you are mailing in a resume, it is necessary to send a cover letter. The cover letter is your way of introducing yourself and your resume. The cover letter should have your name, address and phone number on the top of the page. Then just like a business letter, you have the company's information on the left margin. If in the classified ad it has a contact name out that was your greeting, if it said something like Human Resources address you letter to Sir or Madam. Try not to write To Whom It May Concern, it really isn't necessary to do that. They did not address their ad that way so you don't need to address their letter that way.
When writing your cover letter, mention the ad that you are responding to as well as your interest in applying for the job. Try to summarize your skills into one of two lines in the cover letter but do not recite your resume, that's why you attached it to the letter. Try to keep the letter short maybe a paragraph or two. Explain why in the cover letter that you make a good asset to their company. Try to show a little of your personality in your letter but keep it professional.
Depending on the ad, sometimes it will ask you for salary history and requirements. It is Ok to incorporate these into your letter. Make sure your sentences flow and that there are no grammatical errors. Do not hand write a cover letter, it shows a great lack of professionalism. Borrow a computer or typewriter from someone if you don't have one.
Here is a checklist to follow when writing your cover letter: