Hi Peter,
1. Do I need to write sender's and recepient's address? Yes
2. My wife and I write the letter together, so at the end do I write Mr. & Mrs. Anna Lee or Mr. & Mrs. Peter Lee?
I suggest that you write
Yours sincerely,
then each of you sign it, one signature above the other, as 'Anna Lee' and 'Peter Lee'.
The print your names, below the two signatures as 'Anna and Peter Lee'
3. Do I use semi-colon or a coma when I write Dear Ms. Smith{:} {,} use a comma
4.Which one is the right format to write the admission letter?
Sender address Date Date
Date Recepient address Dear Ms.Smith
Recepient address OR Dear Ms. Smith OR
Dear Ms. Smith
The format on the left. Put sender address and date on the right, then recipient address below that and on the left.
Good luck, Clive