99 Chevy cavalier won't start, anti theft light on

If the anti-thief is on that usually means that the chip is not reconsidered in the key. You can try a reset by from the inside the vehicle manually lock all the doors except the drivers, with key in hand exit the vehicle close and using the key manually lock the drivers door wait at least 3 minutes then using the key unlock the drivers door and see if the light has gone away while you start the vehicle. If that fails then you have a choice of calling a smart locksmith that can reprogram your security system with a new chipped key or have it towed to the dealership and have the system reprogrammed with new keys. Either way will be expensive but the locksmith will be cheaper if they have the equipment. Good Luck.