In order to do this right, you should know something about the company or about the person who will be reading the letter. Really work on a couple of sentences which tell the employer why your special qualifications will benefit the company and how you know you can do the job. It should not take more than two or three lines. Try to match your skills directly with those required by the job you seek. Then say that your resume is enclosed and that it tells more about your special qualifications.
Many people make a big mistake in the closing paragraph and offer the reader their phone number.
This assumes that the prospective employer is required to call them. Tell the reader what your next move will be. Say that you will call and be sure that you do. Keep on calling until you get an answer. The purpose of the first call is to establish a personal link with the prospective employer and communicate your interest in in the company or the position.