What should be included on a cover sheet for a resume.?

It's a cover letter, not a cover sheet. And it should be exactly that -- a letter that states why you are interested in the job, why you think you'd be good at it (the skills and qualifications that you posess that match what they're looking for) and politely thanks the employer for their time and consideration. A good rule of thumb is to carefully study the job announcement itself. Your cover letter should mirror the job announcement. If they ask for good writing skills, then mention your attention to detail and affinity for proofreading (and be sure to proofread your letter!). If the job requires a specific degree, mention that you received it from _______ school/college/university. If they want you to be able to answer phones, mention the volunteer/work experience you have in this area. Basically, whatever they want, try to write about why you have it.

If you are a career-changer, be sure to still address their job announcement. You may have to stretch a little, but you should still draw attention to how your skills and experience can be adapted for your new career.