Using the right sized wrench for the bolt, loosen the pedal from the crank arm.
Something very important to remember: the pedals are threaded so that the cyclist is always "tightening" the bolt as he or she rides. That means to loosen the pedals, you must turn the bolt the opposite way than the crank goes when you are pedaling. While on the right side of the bike, everything is normal, but on the left-hand pedal, it's backwards. There, instead of the normal "lefty-loosey, righty-tighty" phrase that people use to remember which way to turn wrenches, on the left side it is reversed: you will crank the wrench to the right (clockwise) to loosen the bolt.
It is possible that these bolts will be set pretty tight because of all the torque that your powerful legs have been applying to them as you ride. You may have to work at them a bit, but as long as you pay attention to twisting them in the proper direction, the pedals will come free.
One final tip: be sure that your chain is set on the largest chain ring in the front. Just in case the wrench slips and you knock your knuckles against the gear teeth, having the chain there means you'll just skin them a bit instead of receiving a nasty gash.