Best thing to do is search google for more information about cover notes as most people class the initial email as their cover note, in the past I've always gone for the following format:
Paragraph 1: Introduction: Start by stating what job you are interested in applying for and where you came across it
Paragraph 2: About you: This paragraph you should talk about your current status, i.e where your currently positioned, how much experience you've had. Possibly talk about why you want to leave your current position
Paragraph 3: Why your right for the job: This paragraph should focus on why your good for the job your applying for. Talk about what skills you can bring to the team etc
Paragraph 4: Your resume and portfolio: This paragraph should be quite short. Just quickly state how you've attached your portfolio or where the employer can view it online
Paragraph 5: Ending: This paragraph is a good time to end your cover note. Mention that if they require any more information, do not hesitate to get in touch. Also end it with something like 'I hope to hear from you soon"'
Final piece of advice is lay it out like a formal letter, i.e make sure you say Dear sir / madam if you don't know who your sending it too, and end it with something like 'Kind Regards'