By the way, it's a cover letter (not covering letter).
Cover letters are used in circumstances where you want to make a good first impression by stating why you would be perfect for the job you're applying for. You can also submit a cover letter, along with your resume and a few references, to organisations that may not be advertising vacancies ("cold cover letters").
You should search Google. There's a website dedicated to cover letters (and resumes).
Some pointers:
# Use "Dear Sir/Madam" and "Yours Truly"
# Avoid "To Whom It May Concern" - it's rude.
# don't regurgitate your resume
# cite any previous employment positions and experience that is appropriate
# ideally should be less than a page
# describe relevant personality traits that make you approriate for the job
# thank the reader for "taking time away from your busy schedule to read this letter"
# sign it personally. If you printed it in black ink, sign in blue. Makes the fact that you signed it more obvious.