Letter/Mail writing

Dear Teachers,
Could you please help me writing reply mail to the person I recevied Interview call or/and asking to send CV with some other details.
I often struck writing good covering letter and formal response in such situations.

*How to reply to the mail asking for CV for a post. Do I need to type covering letter to that.
*How to write letter/mail to HR in reponse to an ad.

I will appriciate your kind help.