car not starting due to anti theft system

I purchased a used 98 mustang from a private seller and received no owners manual with the vehicle. Somehow the anti theft system was activated and now the vehicle will not start. The key fob performs its functions ie.. locks/unlocks doors pops trunk etc. When i place the key in the ignition all the lights and radio comes on and operates normally when i turn to key over to start the engine it acts as if the battery is dead and clicks and will not turn engine over. I jump started the vehicle and it started but as soon as i turn the vehicle off it will not restart. When the vehicle was running after being jumped the theft light flashed and continued to flash through the operation. i am by no means even a back yard mechanic and on a budget any help would be greatly appreciated
Thank you