4 Runner Tire Pressure Sensor Question..

:banghead: I recently purchased a 2004 toyota 4runner approx 2mo ago. Just recently, the tire pressure warning light started flashing. The 4 tires all have the correct air pressure, this vehicle did not come with a spare tire/wheel. I plan on purchasing a tire/wheel/sensor, when I purchase the sensor from the Toyota dealer, will the light quit flashing, or how do I reset the sensor/ instrument warning light re: the pressure. Also, why did it take 2 mo for the computer to figure out the spare tire was missing?
:screwy:
Thanks for your help in advance..
Darci