If that tire pressure light is flashing when you start the car, it's probably one of those tire pressure sensors acting up. Could be the battery in the sensor is dead, or the sensor itself might be broken. To fix it, you're looking at around $70 to $135, depending on where you are and how much it costs to get your tires remounted and balanced.
If your tires are all way overinflated but if the pressure is okay, then you might have a slow leak somewhere. It could be a bad valve stem, a problem with the way the tire is mounted on the rim, or even a nail in the tire.