* Check the transmission fluid level. If it is low, add fluid until it reaches the "full" mark on the dipstick.
* Check the transmission filter. If it is clogged, it could restrict the flow of fluid and cause the transmission to overheat and malfunction.
* Check the transmission shift linkage. If it is loose or disconnected, it could be preventing the transmission from shifting into gear properly.
* Check the computer for any loose connections or damaged wires.
* Check the throttle position sensor. If it is out of adjustment, it could be causing the engine to stall when you put it into gear.
If you are not able to find and fix the problem yourself, you may want to take your car to a mechanic for diagnosis and repair.