gear shifting tip

I recently learned that when you are shifting gears, you don't want your bike chain to go diagonally because this stretches the chain and causes it to break down quickly. My bike is 21 speeds. Basically this means that there are 3 shifting options on my left handle bar and 7 shifting options on my right handle bar. The left shifter works the gears near the pedals and the right shifter works the gears on the rear wheel.

When you go uphill, you will want to DOWNshift. That makes it easier to peddle. When you downshift your left hand shifter, it moves from a bigger gear to a smaller one. So I think you can imagine that it's a lot easier to pedal a small gear than a big one.

And to avoid a diagonal chain, you can follow this chart:
left gear 1, use right gear 1, 2 and 3
left gear 2, use right gear 3, 4 and 5
left gear 3, use right gear 5, 6, and 7.