First paragraph(self-introduction): mention the position you're applying for, why you're interested, and how you ended up applying.
Second paragraph (body): the argument portion! Explain your experience in relation to the job and why you're a good fit for the position.
Third paragraph (conclusion): thank your reader for their time, and make any clarifications about the position you feel you might want to say
DON'T: repeat your resume word-for-word. Use the chance to elaborate on what you couldn't in your resume.
DO: tailor your letter to each job you apply for. Refer to the job posting, treat it as an essay question, and use your cover letter as a chance to explain why you are the best fit for this particular job. Research the company and department you are applying to.