Just use the gear that will maintain leg revolution (cadence) of 85 to 100 per minute.

The gears on the left change between the three chainrings on the front giving a big jump in gear ratio if used alone. The gears on the right change between the six cogs at the back offering small changes in gear ratio.

Large chainring front + Small cog back = fast gear but hard to push (e.g. for downhill)
Small chainring front + Large cog back = slow easy gear (e.g. for steep off-road uphill)

Try and avoid small + small or large + large as the chain doesn’t like it

The left shifter is for shifting the gears located in the front (cranks) while the right side shifts the gears at the back.

The gears in the front basically determines your speed. The larger the gear (3rd) the faster you are. However, the largest gear also offers the most resistance (hardest to pedal) thus you normally use the 3rd gear in front when you go downhill and the 2nd & 1st gear when you are on flats or going uphill.

The gears at the back give you different combinations of speed vs. resistance in conjunction with your front gears. e.g.
3rd gear in front & 1st gear in the rear will be the fastest but the hardest to pedal while 1st gear in front & 6th gear in the rear will be slowest but easiest to pedal. Every other combination in between results in conditions in between the examples I cited above.

Just practice riding your bike for short distances regularly & you will get the hang of shifting shortly.

Hope I helped.

put the bike on a bike rack and rotate the pedals in different gears, front and back.

you will discover that by changing the gears that the chain is on, the amount the rear wheel turns per rotation of the pedals changes.

The big reason you do this is because we trade power for speed when we change the gearing. going up a hill you need more power, so you trade pedal speed for power at the rear wheel.

you will notice that if you have the chain on the little ring in the front and the big gear in the back, the rear wheel turns the least amount per pedal revolution. this is a hill climbing gear for bad uphills.

And the opposite is also true- the biggest chainring in the front and the smallest in the back give the most amount of rear wheel motion per pedal revolution. this is a high speed gear, like with a tailwind.

the combinations in-between are for the terrain in-between big uphills and fast flying.

In most in-town situations, you will probably initially spend most of your time on the little chainring in the front and just shift the rear, till you “get your sea legs”. later you will spend most of your time in the front middle chainring. Then you use the front chainring for major gearing changes, and the rear gear for minor changes.

Be sure you keep your tires pumped up, your brakes adjusted and your chain lubricated.