Put it in park, then grab the round collar on the steering wheel between the wheel and the dash and turn it with your hands to the left as far as you can. If it will start after doing that, then your problem is likely to be either that the "back drive" linkage is out of adjustment, or the neutral safety switch on the top of the lower steering column near the firewall behind the dash is out of adjustment.
The back drive linkage is a linkage from the transmission shift arm (on the transmission, where the shifter connects) to an arm on the lower end of the steering column inside the engine compartment. Moving the linkage moves the arm and rotates that collar on the column that is ultimately connected to the neutral safety switch. If the linkage is out of adjustment, then the switch with "think" that the transmission is in a different gear than it really is. If you get under the car so you can see, you'll find the linkage rod that goes to the steering column can be adjusted by loosening a clamping nut that the rod slides through. Make sure the transmission is in Park, loosen that nut, then push "up" on the rod as far as you can (rotating the collar to the left), then tightening the nut back down to lock it in there. If it's out of adjustment, that should get you close. Sometimes it takes several tries to get things lined up just right.