Generally, a cover letter should be three paragraphs:
1. First Paragraph - Why you are writing (i.e. which job you're apply for)
2. Middle Paragraphs - What you have to offer such as skills and personal attributes that apply to the job (for example, "as shown by my experience at x, I am very loyal and hard-working..."
3. Concluding Paragraph - How you will follow-up
Here is a website with some examples, but you can find them all over the place if you just google it. It kind of depends on what job you're applying for to determine if it's even necessary, but I think that page has a guide on different kinds of letters for different situations. Good luck! :)