The gears are controlled by moving the chain across sprocket wheels on the back wheel axle. These are changed by the right side twisty grip.
There are also two or three 'ranges' of gears contolled by the left twisty grip.
This moves the chain across sprocket wheels on the pedal shaft.
Dependin on if you are on level road, uphill, or scrambling over terrain, you may find different ranges more suitable. The only way is for you to get out there and experiment.
When you are in the range you want, if you hear a bit of a ticking noise from the pedal area, adjust the left twisty grip a tiny bit until it stops. You may have to do this again if you change gear, as the angle of the chain will alter.