Having a good CV is great, but often employers expect you to have a cover letter to go with it.
Think of your CV like a fact sheet. Without a little bit of explanation it doesn’t really show people who you really are beyond your work experience, school and extra curricular activities. If your CV is a fact sheet about you, then you can think of your cover letter as the actual application for a job.
Your cover letter should be no longer than one page, and should be simple and to the point.
Focus on how the employer can benefit from you and your skill set, as opposed to focussing on yourself. Use sentences like “I believe *Skill A* could be of great use when working within the team environment that you have at *Employer*”.
Look for key words in the job ad that tell you what kind of person the employer is looking for. Take the ones that apply to you and explain how you have displayed/learnt these qualities. For example, if the employer is looking for someone with confidence and who is responsible, explain to them how you are confident and responsible (only if you are of course!) and how you can apply these traits at work.
For your layout, keep it basic, but formal. Be sure to include the employers name and address.