Lower numbers are the low gears, and higher numbers are the high gears. First gear is a low gear. Twenty-first gear is a high gear.Shifting means going from one gear to another. You shift gears by sliding the shifter on the handlebars. On most bikes this shifts the chain onto a different sized ring. On three-speed bikes the gears are inside the hub of the wheel so you don't see them.
Step2
If your bike has three speeds then telling the gears apart is easy, because there's only one shifter and it's labeled 1-2-3.But if your bike has 10 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 labeled 1-2-3, and your right shifter will be labeled 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.
Step3
If you're going uphill and it's too difficult, shift down. If your legs are spinning the pedals way too fast (it's too "easy") then shift up. ( Uphill = shift down, Downhill = shift up.)
Enthusiast 
