Beijing’s subway system is reliable, convenient and user friendly. Find yourself a good map of the network (usually provided by your hostel) and away you go. Prices ranged from 2RMB/ AU$0.40 to 6RMB/ AU$1.20 depending how far you travel.
We bought individual tickets each time we travelled, but if you are there for an extended period you can by a rechargeable card which has lower fare prices.
If you get stuck, here is a map of the Beijing subway system (click to enlarge):
Sourced from http://www.travelchinaguide.com/images/map/beijing/beijing-subway.gif