@sadieme, I hope this is what you refer to when you ask about gears. Sorry if I'm telling you something you already know. So you have 3 chainrings on the front and they are controlled by the shifters on the left side of handle bars. Changing the chain using these chainrings in the front make BIG changes in regard to peddaling from easy, medium hardest. The large or outer chainring is the hardest to pedal, the middle chainring is a little easier to pedal and the smallest chainring (many call it a "granny gear"-very inappropriate thing to call it in my opinion) is the easiest to pedal.
The shifters on the right side of the handle bars is for the rear cogs (I remember this by thinking "right for rear"). Shifting these gears make smaller, often subtle changes. These gears work just the opposite direction from the front: In the rear the big cog (gear) closest to the frame of the bike, is the EASIEST to pedal, and the tiny cog (away from the frame) is the hardest to pedal. So the gear combination which will be the very hardest to pedal would be the big chainring in the front and the smallest in the back. The MOST easy gear combo. to pedal would be smallest chainring in the front and largest in the back. There are various combinations which will feel almost identical, and that is because there is a lot of overlap. There is no "right" or "wrong" gear. If it's too hard to pedal, shift down to an easier gear; or if your feet are spinning like crazy and you're not getting anywhere, shift to higher/harder to pedal gears. You will see that when you're in the hard to pedal (higher gears) you move more distance per pedal revolution; when it's very easy to pedal you won't move as far with each pedal stroke.
Hope this helps.