The first thing you can do when your key switch won't turn off is to remove the positive cable from your battery. You will need to get the key switch ignition changed.
Could be you need a new fuel idle motor. Or you need new spark plugs
Underneath the passenger side rear floor, under the vehicle. It is round, cylinder shape.
transmission wont shift from low to high very good
Trany shot replace
Try looking at replacing the crankshaft position sensor, i had a similar problem with my 92 lumina and i traced it back to a bad crank sensor.
You must turn the wheel back and forth while turning the key. This will unlock the key and allow it to turn.
neutral switch
Been having this problem myself with the exact car. It's the stupid "chip" key. Needs to be replaced/reprogrammed which can only be done at a Chevy dealership and can be quite expensive. Good luck.
I had a similar problem turns out the Alternator was bad.
sounds like pluged catalitic converter or egr valve
Older Chevy's have a steering lock when the key isn't in the ignition.
did you check your battery and alternator?