Go to where the beginning of the a/c coil drains. If this is a combined furnace/a/c unit, then your coil sits inside of the plenum above the furnace so the fan can blow through it and through your duct work.(not on the outside)
Coming out of the plenum where the coil is will be your copper pipe(covered with black insulation) that moves freon to the condensing unit outside the house. In that same area will be about a 3/4" or 1" pvc pipe or hose coming out of the plenum that goes to a drain somewhere or into a little pump basin where it is pumped to the outside. Take the pvc pipe/hose loose and poke your wire into the plenum a little ways. These drain holes plug up with crud fairly often. Clean/blow through the pipe/hose.
If the a/c drains into a pump basin that may be attached to the side of the furnace and it is overflowing, then post back with that info. They have a float valve and small pump and all of which may need to be taken apart, cleaned, or replaced. Post back.