"MAX PSI: ##" is usually written on the side of your tire - where, "##" is the Pounds per Square Inch (PSI) that the manufacture recommends you put into your tires.
The correct way to fill your tires is to use an air compressor (at your gas station for example) and fill to a few pounds just about MAX PSI, then check the pressure and release some of the air until it is exactly at "MAX PSI".
By having your tires under inflated, you may be loosing on gas mileage, causing more pollution and wearing your tires out faster and passably un evenly...
By having your tires over inflated, you may be wearing them out more quickly and you may be more at risk of traffic accident because your stopping distance will be reduced because the tires might not be able to grasp the road as well.
The average tire pressure that is printed on the wall of your tire for a standard automobile is between 25PSI and 45PSI ... ALWAYS CHECK WHAT YOUR TIRE'S RECOMMENDED PSI SHOULD BE SET TO... if you cannot read what is written on one tire, check another that is on your car - 99.9% chance that all the tires on your car are the same (as they should be)
Some racers like to relieve 5 to 8 lbs of PSI from the tires where the engine gives most torque (FWD or RWD), to make them more "STICKY" - However, this works just fine for the track, but for daily driving, you should always inflate to manufacture's recommended PSI