First, let's get our terms straight so we're on the same track
If your bike has three speeds then telling the gears apart is easy, because there's only one shifter and it's labelled 1-2-3. You can skip the rest of this section and go on to the next one.
But if your bike has 12 or more speeds then it's just a little bit trickier, because you have two shifters....Let's say you have an 18-speed bike. Your left shifter will be labelled 1-2-3, and your right shifter will be labelled 1-2-3-4-5-6. This means that for each number on the left, you get six different speeds on the right, for a total of 18. Here's how it works:
Shifting puts the chain on a different ring. The left shifter changes the ring where the pedals are. The smallest ring is 1, the middle ring is 2, and the biggest ring is 3. When you downshift with your left shifter, you're moving to a smaller ring.
The right shifter changes the ring on the rear wheel. This is opposite of the front set: On the rear wheel the biggest ring is 1, and the largest ring is 6.
You don't have to worry about the ring sizes if you don't want to, you can just look at the numbers on the shifter. You can downshift with either shifter, moving it from a higher number to a smaller number. You get a bigger change when you shift with the left-hand shifter than when you shift with the right-hand shifter.
Now that you know what the terms mean, let's see how to use our gears
Here's pretty much all you need to know about shifting gears:
Here it is with more detail:
So you've learned the basics: Uphill = shift down, Downhill = shift up. But you have an 18-speed bike and you're wondering which shifter to use, the left one or the right one?
The answer depends on how much change you need. If you need a big change, use your left shifter. If you need a small change, use the right one. As you ride you'll get a feel for whether you need a big change or a small change.
Another thing: Try to keep the chain in a sort-of straight line between the front and rear sets, rather than going at an angle from left to right. For example, in the very lowest gear the chain will be all the way on the left on both sets. In the very highest gear the chain will be all the way on the right. What you don't want to do is to use the left-hand ring in the front and the right-hand ring in the back, which makes the chain go diagonally -- that stretches the chain and wears it out. You also don't want to use the right-hand ring in the front and the left-hand ring in the back. Same thing.
If you're in the easiest gear (left-hand on both rings) and you need to upshift, and you've moved the chain on the rear set to the middle, and that's not enough and you still need to upshift, don't keep shifting the rear set. Instead, shift the front set from the left to the middle (1 to 2). That keeps your chain nice and straight.
Let's have a look at our gearing chart. The green boxes are the combinations you'll use, and the grey ones are the combos you'll avoid.
|#L/H Chainring||#Middle Chainring||#R/H Chainring|
So yes, you won't use every gear available to you. You're not supposed to. When you get an 24-speed bike vs. an 18-speed bike, the point isn't really that you get more gears to use, it's to give you smoother and better gear changing - smaller steps between the cogs on the rear set.
I hope this helps, and have fun with your gearing!