2002 Dodge Intrepid Just Bought Used 02 Intrepid Test Drove Problems Everything

Just bought a used 02 intrepid, test drove no problems everything works well. I locked the doors with the inside switch, and came back to pick up the car. The car had no remote only the key. When I unlocked the driver door an opened it the alarm went off, the could would start and run but I cannot disarm the alarm. What do I do???