You have to buy the school's envelope with the school crest and address on it, no further address is required on the envelope, just post it.
You will need the registrar's address if only you are using a different envelope apart from the schools envelope.
In the process of posting, just send it to the post office. The official workers at post office will just write your name and you sign. Make sure you write your name,first choice of program and your number on the envelope.