Help with the bicycle gearing
Pedalling efficiency/difficulty is related to the ratio between your front and rear gears.

Front : 


The big crown is the hardest one
The small crown is the easiest one


For rear gears this is the opposite.

In general, you should avoid crossing the chain, which is:


Putting the small front and rear gears at the same time
Putting the big front and rear gears at the same time (especially this one that might break your chain)


After that, the terrain you are riding pretty much determines the front gear to use:


For flat, you should be either on the biggest or on the middle front gear
For uphill, you should use the smallest front gear
For downhill, you should use the biggest front gear


Then, you tweak the difficulty with the rear gears.

For reference, the ideal number of rotations per minutes (of the cranks) is 90 in road cycling (that looks fast, but is efficient).