As the first thing that many of your employers will see, a cover letter is a very important document because it must communicate a good first impression while also notifying the company about the role you wish to play with them. Often sent along with your resume, a cover letter must provide a good introduction to who you are, rightfully complementing and not simply reiterating your resume or CV.
Highlight your strengths. A cover letter is, by its very nature, concise and to the point, so instead of taking up the whole page outlining everything you have previously done, or rather talking about how much you care for your prospective position, use the space you have to talk about your strengths. One of the most important things that you put on a cover letter should be reflected in your language. Nearly every company you work for will involve at the very least some sort of interpersonal interaction, so presenting yourself as an approachable person that the company can relate to is important. This should furthermore be reinforced by your spelling and grammar, whose impeccability speaks to your ability to achieve perfection, even if it is in small doses.