The best way to write a cover letter is to have the job notice with job duties and requirements right in front of you. Start out by seeing if you can find the name of the HR person in the company and write directly to them. Start out by stating that you saw their notice for the job (give job title) in/at (state where you saw it and on what date). Then state that you felt that your abilities and experience make you an ideal candidate. Then use bullet point not paragraph form to list Job Duties (as listed) with your matches for the ones stated beside in a two column format. If you do not have experience put fast learner/eager to learn. Do the same for Job Requirements: Like high school degree put GED or whatever applies, knowing certain programs put those similar you know or are willing to learn. Then conclude with a strong confident statement asking for a face-to face interview so you can learn more about how you abilities can be immediately beneficial to the company. By making the HR's job as easy as possible, your cover letter and resume will end up on top the pile. Also, check and double check for spelling, grammar and layout errors.

I usually just google "cover letter example" to see what I need to do.