Caths17
Firstly welcome to The Hub - look forward to seeing you posting more - good fun postings to add to right now too.
As for business letters, in the UK - as far as I'm aware, and what I've always done is:
Mr Smith
Snow Company
17 Winter Park
LONDON
TH3 P1TS
Beginning with:
Dear Mr Smith
Ending with:
Yours sincerely
or:
Attention (or FAO...for the attention of) Mr Smith
Snow Company
17 Winter Park
LONDON
TH3 P1TS
Beginning with:
Dear Sir
Ending with:
Yours faithfully
Hope that helps - but may have just confused you more, in other areas of the world, it may well be different.
Pam 
Peer Moderatorwww.Aberdeen-Secretaries.co.uk