how to apply for a visa for a nigerian coming to attend a conference @ hong kong and charges involve

I think you can directly apply for ja visitor type from the HK Immigration Department. It costs around HK$160. The address is as bellow:
Receipt and Despatch Unit
Hong Kong Immigration Department
2/F, Immigration Tower
7 Gloucester Road
Wan Chai, Hong Kong

Usually, the required documents are your passport, filled ID1003A form together with a recent passport size photo, a copy of your passport information page, flight arrangement and financial statement (required for some nationals).