I have a 1994 chevy s-10 pickup. Its a stick shift and it wouldnt let me shift gears. I can change gears when to truck is off but when i try to start it while its in gear it wont start. I dont even know where to start to fix this problem.

Start by checking the Clutch. You can shift gears when the motor is off because there is nothing moving but when the engine is on you have to have the clutch to get things to meld right or you will damage your vehicle. if it's not the clutch assembly it's going to be the transmission. The neutral safety switch will keep it from starting if the truck is in gear and the clutch isn't engaging.