Since you asked - this is how to ride a bike with gears.
First of all, some technical info on how they work:
How they work - bicycle gears
Your gears (sorry I don't know all the technical terms) use them like this:
Now the dial on the left controls the derailleur for the pedals. For starters it's probably best to keep it in 2. 1 is for hills (lots of easy fast pedalling) and 3 is for downhill/high speed (less pedalling but much harder at slower speeds)
The one on the right controls the back derailleur. And that's the one you'll be changing much more often. Now the extreme combinations are not advised to you don't actually have all the permutations at your disposal, and there is some duplication: 2-4 might feel the same as 3-1.
For example don't do 1-6 or 3-1 as that might stretch the chain a bit too much.
Anyway - start with 1 or 2 on the left and leave it there for starters. Remember that you can only change gear while the pedals are moving (see last bit about stopping)
To start from standing, you want to be in a low gear on the right, so start with 1-1 or 2-1. Then as you pick up a bit of speed and your legs go faster and faster switch the RIGHT one up a notch - to 2. And so on. It's generally a good idea to wait to feel the chain move into the new position before switching again.
If you're cruising on 5 for example, and the comes a hill, gradually switch the right one back to 4 - 3 - 2 until it is comfortable.
When you come to a stop make sure you switch right down before stopping to make it easier to pull off again.
My bike only has 6 gears, so I can't describe good techniques for one with 18 like yours. I use 1 to pull off, 3 going up hills (errm, railway bridges actually) and 5/6 to whizz fast on a flat or downhill.
Good luck I'm sure you'll get the hang of it!