Dear Mr. / Ms. (Last name of Manager):
This should include the job position that you are applying for and how you have heard of the vacancy. Include a brief and strong statement on why they should hire you. If you were referred by an employee of the company, make sure to include that here.
You should mention the key strengths that you possess that make you fit for the job more than anybody else should. Make sure to include facts to support your strengths. Do not repeat information written on your resume because this will be unnecessary and redundant. Make sure to use this little space to expound on your experience and skills. A job application letter should only be contained in one page.
Thank the employer for taking the time to read your letter and make sure to include your most available contact detail. Mention also on how you intend to make follow-ups about the job offer. It can either be a week to a month depending on your preference. A shorter follow-up period is advised to make them feel that you are indeed interested to work for them.