Once the letter is written, add a line space and use either "Yours faithfully," or "Yours sincerely," before your signature. There are several schools of thoughts as to when it's appropriate to use "faithfully," or "sincerely,".Follow this rule:"Yours faithfully," should be used when the name of the recipient is not known (so, in a letter with an introduction of "Dear Sir/Madam:").
We will all go through writing a business letter at least once in our lifetime. It does not matter if you are an employee, an owner, a stay_at_home mom (or dad) that has her own business at home; we all have to go through writing it. It cannot be avoided!