Requirments for applying S1

Hello Mrs lin,
I'm student and I want my wife to get S1 visa from my home. Requirement no.3 that said " An invitation letter ......" is it an official letter written by myself or by the university and how can I get this invitation letter?
-Financial source of living expenditure is required in my home or when my wife arrive China and from where we can get a notice letter to prove that?
-who is the inviting individual mentioned in section c.
-Requirments no.4: A photocopy of the inviting individual's ???
- last question: is physical examination required when applying to S1?
Thanks in advance!