Visa Applicaion in Korea for an Ethiopian

I'm an Ethiopian by citizen, and now staying in Korea as graduate student.I have been invited by one Chinees company for an official visit from July 15-18th.My question is, should I apply the official invitation letter only or both the flight ticket and the invitation letter?I still didn't buy the ticket,and I need to apply since my ticket may delay some how.The second question is,I'm staying in Daejeon city in Korea.Do you have a travle agents in Daejeon city?the third question is,is it possible to get it online if i send all my credentials online?I would be happy if you send me through my email adress.

Mant thanks,
Teshome Worku