what type of visa? What type of invitation letter?

I am a Nigerian, just finished a graduate degree in 3D animation in U.K. I got an admission for an unpaid internship by a private company in PRC for between 3-6months. This will expose me to good hand -on- skill, and improve my CV. I intend soon to finally come home to Nigeria, just before my U.K student type expires, and would apply for the Chinese L type immediately. Please, what type should I apply for? What type of invitation letter is required from the invitee? Any other important information about the required invitation letter.
Regards.
Lami