I have a consignment I need to pick up in China. It costs a lot of money to cargo it to nigeria, some friends have advised me to go to china myself to pick it, that I will only need to pay for the excess on kg on my flight .which will be cheaper. Is this true?

Also If I need to go to china, what documents do I need to apply for a business trip to china

You'll need a letter of invitation from a reputable company in china, bank account statement, proof of your biz registration, proof of ownership of a landed property......The list can go on and on. The reason is because the chinese govt is trying to reduce the influx of nigerians into china. There are agents in naija who can handle your visa for you. Seek out a good one and make sure you don't pay him any money until the visa is out. Best of luck. By the way i'm in china.

Pls wat is d rate of naira to china currency and how i can school as well as doing per time job in china

Its 25.6RMB or thereabout to the naira.
And please there is no part time job in中国。so just keep that in mind.
You can only do Biz