Before you can write a cover letter, you might want to check on a cover letters template on the internet. Just like writing a resume or something else related to your professional work life, you really have to be distinct when you are writing a cover letter. If you are not distinct, there is a chance that the one reading the letter would end up saying that you are nothing but a plagiarizing person. There is nothing worse than being seen as a plagiarizer, so you have to be really distinct when you are writing your CL.
That does not mean you can stray too far from what is generally working
Being distinct does not mean being very different from what is working, now. While you need to distinguish your CL from other CLs, you have to stick with a template that is working. If you deviate too much from it, the recipients of the letter might think that you are not being serious with your letter and might throw it away the instant they open the envelope.
Different ways for different mediums
You also have to understand that writing your cover letter in the form of email and hardcopy requires you to have different ways of writing the aspects of each medium. The internet is ripe with cover letters template that you can learn from, so get to learning this moment if you want to create a good one.