There is no hard and fast rule of selecting gear. But unlike a car, gear in a
bike is never on neutral!
You should change gear only when the bike is
moving (not when it is stand still). Some folding bikes don't have front
gears (only on rear). For normal riding on level ground, you may leave the
gear M-3 setting. For hill climbing better to use M-1 or M-2. For very steep
hill, L-1 or L-2 can be used.
The pace at which rider pedals is known as "cadence". Most regular cyclists
maintain a cadence of 50 - 100 per minute.