I am planning to visit China and I need some information for visa application:

I am planning to visit China and I need some information for visa application:
1. How long in advance do I need to apply?
2. Do I need to visti your embassy or I can send all documents?
3. If I have to visti your embassy do I need to book an appontment in advance?
4. I will have an invitation letter from a Chinese university. The letter has to be the original or they can send it to me by fax?
Thanking you in advance