Spray some WD-40 and let it sit for a while. On the side of the bike you want the pedal off, move the crank arm to the 6 o'clock position and put your wrench on at the appropriate angle. If you're turning the wrench clockwise, the wrench should point to the 3 o'clock position. Counter-clockwise, 9 o'clock. Tap the wrench in the correct direction with a hammer. Next time use some grease.
Good luck!