Ride the bike at a pace that's comfortable to you. Cadence will improve with fitness. Counting seconds in your head while monitoring crank rpm will also give you a reasonable good idea of cadence...say XX rpm in 30 seconds.

I usually run 53T front, and 19-15T rear on a 700c tire. My nominal cadence is 65-75 rpm. Higher cadence burns more energy, but it also provides a more stable pace if you encounter a sudden burst of head wind. Lower cadence conserves energy. The downside is frequent shifting if you ride in hill country.

You are the MOTOR. Shift to overdrive if you want to save gas. Downshift a few gears if you want to pass or climb a hill. Novide riders try to maintain the same cadence all the time. That's a mistake. A car will normally run between 2000 and 2500 rpm. Some with drop as low as 1650 rpm at freeway speed to save gas.