You should mark your piece on the finished side.
Place a straightedge next to the mark and score the piece on the finished side - you only have to score about 1/16" in. A drywall t-square is helpful but you can get by with any straight edge if you are careful.
Lift the sheet upright and then tap the piece on the unfinished side with your knee while holding the two sides. It will snap and then you can fold it in toward you. It will be folded and standing vertical.
Cut along the unfinished side at the fold - this will complete the cut. Be careful that the two halves do not rip away from each other before you can finish the cut. Having someone assist you here is helpful. More information is given at
Drywall Installation - Cutting and Installing Drywall