Pontiac Repair: key will not turn to start position, pontiac sunfire, jumper cables

Question
I have a 2000 Pontiac Sunfire. When i put my key into the ignition, the key will turn into the first 3 positions but will not turn into the 4th start position.  I have tried starting it with a battery and jumper cables hooked up to the starter, and it worked the first couple of times, but this will no longer work to start the car either.  I have been told that it is the key lock cylinder and i have also been told that it is the ignition switch, i am not sure what it is.  Any information or opinion on it that you could give me would be wonderful and greatly appreciated.  thank you.

Answer
Hi this could be the ignition switch. it also could be the starter solenoid. or the safety switch on the shift lever. make sure the battery is fully charged up. and has good connection at the battery terminal. To check this take a test light, have someone to help you, let them trun the key, while you check for fire going to the little wire  that is conected to the starter. If there is no fire then it could be the switch. If you do have fire then it would be the starter. Also check your fuses.