I have an invitation letter already, what else do i need to apply for a visa?

Biotumelo, if this invitation letter is from the company, it is useless. You should ask your employer to get a work license and an invitation letter from a duly authorized unit, namely, the foreign affairs office or some other authoirzed agencies. Besides, you need to prepare your passport, a copy of your passport's photo page, and an application form attached with recently-taken color bare-headed photo. Sometimes, the medical report is also needed.