1) Dear Sir or Madam,
If you do not know the name of the person you are writing to, use this. It is always advisable to try to find out a name.
2) Dear Mr Jenkins,
If you know the name, use the title (Mr, Mrs, Miss or Ms, Dr, etc.) and the surname only. If you are writing to a woman and do not know if she uses Mrs or Miss, you can use Ms, which is for married
1) Yours faithfully
If you do not know the name of the person, end the letter this way.
2) Yours sincerely
If you know the name of the person, end the letter this way.
3. With kind regards
Commonly used.