Select whichever gear allows you to pedal comfortably. This may change many times on the same stretch of road. It is generally recommended to pedal at a rate of between 80-100rpm, it'll feel like you're pedalling really fast, but it's less tiring on the legs. If you are spinning too fast, change to a smaller cog, if it's getting too hard, change to a larger one.
On flat ground, you'll probably be in the middle of the front chainrings, then change the rear gear to suit. You can use the full range of rear gears from the middle ring.
The large front ring comes into play when going downhill. When on this ring, avoid using the largest rear cog.
The smallest front ring is for climbing steep hills. Avoid using the smallest rear cog.
As you get stronger, you will find you can push higher gears than before. After a while, you might find that you are using the big chainring much more on the flat. Gear selection is something you get used to and with some practice you will be able to predict what gear you will need at any given point.