here is a run down of gearing
The Gearshift on the left controls the gears in 3sets the front gears , Where as the GearShift on the right controls the rear gears individually.
For example
If your bike has 21 gears, Then the gears are divided into 3 groups of 7 gears.
When left GearShift is on 1, you use the Right GearShift to move through gears 1 - 7
When left shifter is on 2, you use the Right GearShift to move through 8 - 15
When left shifter is on 3, you use the Right GearShift to move through 16 - 21
I Hope that Make sense
The lower the gear, the less resistance. Use a lower gear for going up hills.
When start descending the hill gravity pulls you down making you go faster DOWNHILL, Shift into a higher gear so that you can to pedal along and go faster.
You will find that you using your right shifter more than the left I'm not sure if I
I hope that i made sense and that i explained it good enough