Step 1. Read the job ad carefully. Inside that ad you're going to find key words. When the employer wrote the ad he described his dream employee.
Step 2. Make a list of the key words within the add. If the ad says something like;
Brand new pediatric dental office looking for a front office staff member with dental office experience. This person should be a team player who can ensure the smooth and efficient running of the practice as a whole. The ideal candidate will be friendly, enthusiastic, and have a positive attitude. Also, they should be a quick-learner, flexible, and have excellent organizational and customer service skills. Tasks include welcoming patients, scheduling appointments, presenting fees and discussing dental insurance benefits, processing dental insurance claims, tracking and calling patients with incomplete treatment, and all other front desk tasks. Great opportunity for a highly qualified candidate that possesses strong leadership skills. Salary based on experience.
The key words here are; team player, efficient, enthusiastic, positive attitude, flexible, excellent organizational and customer service skills, strong leadership skills. So you'd want to hook the person who reads your cover letter by using these words. This is what they are looking for, so be that person.
Step 3. Check out the website if possible. Google their name, email address, or phone number to find it. There might be more key words on their website you can find to use in your letter.
Step 4. Make your first sentence or two a hook. That means say something right up front to make them read on. Don't start with "I would like to apply..." They know that already, you gave them your resume. Try, "I'm looking for a position that will appreciate my positive attitude and excellent customer service skills." In that first sentence you've incorporated two key words from their wish list.
Step 5. If you can say something personal about their business, add that, too. The above ad if for a pediatric office so any experience you have with kids would be good to mention. If it's a construction office, you can admire something you saw on their website.
Keep it short and to the point. All your skills should be listed on your resume, so don't list them all here. A highlight or two that coordinates with what was asked for in the ad is fine, but most employers don't have time to read a book.
If you're not that great a writer google cover letters and copy them, BUT don't forget to add your keywords and personalize it to make it stand out from the crowd. Use the bones of those online letters, but you need to make them your own and tailored to the job for which you're applying. Every job you apply for should have a different cover letter that's been modified to reflect the job, key words, and how your skills would match.
Good luck! Go get 'em!