During the summer months you should inflate the tires to the recommended pressure on the tires.
The recommended tire pressure should be stamped into the sidewall of the tire which will tell you the recommended air pressure to inflate the tires too when the weather is cold or hot.
Most vehicles though especially cars have a recommended tire pressure of 32 or 35 PSI which is a good air pressure for summer time.
But always look on the sidewall of the tire or in your vehicles owners manual to get the correct pressure to be safe and improve fuel mileage.