In general it's best to keep your tires pumped up to maximum tire pressure.
For beach cruisers, the best pressure is usually 40 psi.
Hybrids can go up to 65 or 80psi.
The tire pressure range is written on the tire side wall.
Keeping the tires inflated to maximum pressure will make your ride easier.
You can also ride with reduced pressure. This will make the ride softer, but will result in a slower, more difficult ride. For more technical riding on loose or slippery terrain, lower tire pressure can also increase traction.