I ran 2 new 20 amp circuits, one for lighting, the other for receptacles. I ran two 12 gauge wires for the hots and one neutral and a ground. I hit the first receptacle, which is a GFCI, and connected it to the line side. I ran the other plugs to the load side of the GFCI. I pig tailed the neutral to the neutral wire on each receptacle. The neutral is the same for the lighting and the GFCI. The problem is, I have 120 v @ each plug. When I hit the switch for the lights, the breaker trips on that circuit and the GFCI trips. I believe there is something wrong with the neutral wiring. In addition, the house isn't grounded (old wiring). Will the GFCI even work in this application. Thanks

Yes, GFIs do work with no equipment ground.

You cannot share the neutral on the load side of a GFI. You need a dedicated hot and neutral for load side.