A cover letter is not a college admissions essay, nor is it intended to be a writing sample. It’s also not supposed to be a version of your resume in paragraph form. To put it simply, a cover letter serves as a preface to the rest of your job application. Think about it as the introduction to the story you’re trying to tell in your resume – how are you going to frame the rest of the information you’re putting forth in your application?
To write a good cover letter, all you really have to do is concisely explain why it makes sense for you to work at a particular company. What skills do you have that align with the most essential skills a hiring manager is looking for? Why are you interested in the company or role on a personal level? Are there any unique circumstances that make you a stand out candidate that can’t be demonstrated in a resume? And that's really it. If you can answer these questions in half a page or less, you’ll be in good shape!