I guess you ahve to align all the edge vertices on your middle axis? If that's your problem, select all of them, (you might want to do this in the front view and turn x-ray on to make sure you don't accidentaly select other ones) now go to the scale tool and and scale towards the center in x direction. If you check your help line it gives you numerical information. Scale until it shows 0 or something very close to it. Now with your vertices selected hold down the x key to activate grid snapping switch to the move tool and pull them toward the center axis.
They should now all be nice and alligned, and you model ready for mirroring.

I hope it helped