It is wise to remember that higher gears put more resistance on the pedals but pedaling it on a flat road makes you go faster without using much of your energy to attain speed. However, on a steep climb, you might find it difficult to climb on a high gear and may strain your leg muscles.
On the other hand, lower gears make the bike easier to pedal and can make the bike go faster. However, pedaling fast on a lower gear to increase acceleration would tire you easily. But on an uphill climb, a lower gear would make the bike easy to pedal going up. Although in this case, the ride going up would be a bit slow.
Ideally, use the front gear number 1 for steep hills, number 2 on flat ground and slightly elevated terrain, and number 3 for going downhill. The front gears however, should be used in combination with the back gears.
Experts advice cyclists that a good combination for going uphill is to shift to the small chainring, in the front gear with the largest sprockets at the back (e.g. 1-3).
For everyday terrain such as on a regular flat road, use the middle gear. This is achieved by shifting to the small chainring (double/compact) or the middle chainring (triple) in the front gear with the middle sprocket (3-6) in the back gear.
For going downhill, you can use a combination of the big chainring in the front with the smallest sprockets (e.g. 4-7) in the rear. This combination is also perfect for riding long distances since each turn on the pedal allows you to cover greater distance.
You might want to avoid combining the smallest chainring in the front gear with the smallest sprocket in the rear. It is also not wise to combine the biggest chainring in front with the biggest sprocket in the rear. These combinations cause cross chaining which wears out and damages your chains and gears quickly.