sddsddean is correct but, if you can't find the handbook for your car, there's often a chart displayed next to the air pump at garages. This shows the recommended pressures for most makes and models.
If you happen to find yourself driving a car for which you don't have the information available (e.g. if you borrowed someone else's car for a few days), the most important thing is to ensure that the pressures of the two tyres on each axle are the same.
I used to deliver cars for a living and, in the absence of any other information, I'd always use 30psi for both front and rear tyres. However, my current car has recommended pressures of just 23psi, front and rear, so it's always best to use the recommended pressures when you can.
Chris