Welcome to the forums! I am neither a teacher of English nor a native speaker. Here's my two cents.
It's cover letter but not covering letter, and CV but not cv. Personally, I think a successful cover letter will cover the points as below:
1. Standard layout of letter writing.
Formal Letter Writing - Articles - UsingEnglish.com
Note 1: Avoid using the addressing -"to whom It May concern". Use "Dear Mr./Ms +surname" or "Dear Sir or Madam".
Note 2: Regarding to ending letter, write "Yours faithfully" if you don't know the name of person; write "Yours sincerely" if you know the name of the person.
2. Tell why you write to them and how you know the opening. You can make reference to the previous meeting with your job interviewer here.
3. Tell them the reasons why they will hire you.
(1) Who are you?
(2) How you fit in the position? Outline Your qualification, experiences, highest diploma etc.
(3) Why you are interested in their company? What can you bring to the company if they hire you?
(4) Outline your personality, eg. hardworking, outgoing.
4. Make your cover letter terse and efficient, usually no more than a page.
5. Try your best to make your cover letter professional. Avoid any Internet chatting language, eg. i m happy. nice to meet u; good 4 u.
If you like, you can post your cover letter here for teachers or other forum members' suggestions.