Bike Gears?

So I just got my bike fixed for the summer and I want to know how to use the gears while I ride. It has the 123 on the left and 1234567 on the right, what does that do? I understand it has to do with how the wheels turn to your peddling, but it doesn't quite make 100% sense to me. If you can really break it down and explain it: how it works, when to change gears, where to shift it to depending on the hill size, etc. I'd really appreciate it. Thanks!Bike Gears?
The left gears are located at the pedals and make 'major' adjustments. The right gears are in the rear wheel and make 'minor' adjustments. They overlap to some extent (for example, left 2 and right 1 is a lower gear than left 1 and right 7). You want to change gears so that you are pedalling at the same pace and effort. Uphill, reduce gear so you aren't putting strain on your leg muscles and knees. Downhill, increase gear so you aren't just spinning the pedals uselessly, and so you can achieve good speed. On rough surfaces, knobby, bumpy or muddy surfaces, reduce gear. Increase gear on smooth surfaces.

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