To manually level the bed you can use a piece of paper and rest it between the nozzle and the print surface. The Z axis should be in the home position. Slide the paper between the nozzle and print surface, if it is loose adjust the bed leveling knob that is closest to the nozzle until you feel the paper being gripped slightly. Repeat this for all four corners. You may need to repeat this as adjusting one knob may interfere with the level from another.
There are several ways of automatically leveling the bed. These are slightly more complicated to implement. You will need to update the firmware for them to work. See the Auto Bed Leveling page for more information.