It used to be 30rmb for 4000mins (dunno the current rate, probably the same); you can buy credits in some convenient store but mainly you want to buy credits off net bar (网吧). However the policy may depends from one net bar to another and some crappy ones may ask you to give the money so they refill for you; should they be trusted, frankly I dont know but I would take the risk. Ask them to give you refill instead.
You have the link and the process to recharge explained in detailed in some stickies of this forum.
田畕畾畕田 - Mage Herborist/Miner/Grinder