A cover letter should always be sent off with your CV, and it’s a chance for the interviewer to get to know you a little bit, and see why you’re suitable for their position.
It doesn’t have to just reiterate what your CV says, you can include information about the company, and this will show that you’ve done a little research. You can aim to tell them how you can improve the company and help it develop, because that’s what they’d be employing you for.
With a cover letter, you have to get the basics right. There should not be any obvious grammar errors, while it should read quite formal. The majority of your information is going to be in the CV, so don’t waste too much time going over those points, and use the cover letter to persuade the company a little more.
There are many cover letter templates that you can find online, and whilst you shouldn’t use the same template completely, it can give you a few ideas of how to get started.