We will all go through writing a business letter at least once in our lifetime. It does not matter if you are an employee, an owner, a stay_at_home mom (or dad) that has her own business at home; we all have to go through writing it. It cannot be avoided!
A winning cover letter generally consist of three parts which amount up to three paragraphs. In the first paragraph, you will detail why you are writing the company. This will provide context to your intentions for writing the company, which is to apply for a new job. Also, try to provide clues as to where you found out about the job posting and what got you interested in applying. The second paragraph serves as the gist of your cover letter. Here is where you explain what you can offer and what skills matched the job requirement. If you have other skills or achievements, make sure to point them out by referring to your resume. Make sure you can convince them to read your resume.