ok ok, lets keep it super simple - how hard it is to pedal depends on the combination of sprockets the chain is running around. generally speaking, the big gear on the front, connected to the pedals, or cranks, is called a chainring. there are usually 2 or 3 of these. generally speaking, the bigger the front ring, the harder it will be to pedal. so like on flats you usually roll around in the big ring, or "3" if your shifter has numbers printed on it. if you're climbing a hill you might go down to 2 or 1, which are progressively smaller, and provide easier pedalling.

on the rear, the opposite is true - bigger gears mean easier pedalling, smaller gears mean harder pedalling. on a bike like yours, "1" will be the easiest to pedal, and the largest actual gear with like 28 teeth, "7" will be the smallest gear with like 11 or 12 or 13 teeth and will be the hardest to pedal.

you pick your "gear" by selecting both a front AND rear gear, which gives you the overall "ratio." because there are 2 or 3 gears in the front, but 7 or more in the back, it is easier to shift the rear because the steps between the gears are smaller. this also makes it easier to fine tune your cadence, or how fast you are spinning your feet. some people like to spin their feet faster or slower depending on conditions, resistance, personal preference, fitness, etc.

so think of this way - you select a "bucket" of gears up front depending on how hard it is to pedal down the road. most of the time on flats you'll be in "2" or "3" only shifting down to "1" in the front when you climb hills. within that "bucket" of gears up front, you will do most of your shifting in the back, using your right hand, between gears 1-7, to make small adjustments in how easy or hard it is to pedal, based on your preference.

generally speaking, keeping the resistance light, with low gears, and spinning your legs fast, is less tiring on your muscles than using large gears, pushing hard or "mashing" while pushing your legs slowly around, regardless of how fast the bike is travelling down the road. the tradeoff is that spinning requires more cardiovascular fitness. if you are really fit but not strong, you would prefer to spin your legs fast. if you are not fit but have strong legs, you may prefer to spin your legs slowly. generally speaking, as you improve both strength and fitness, you will begin to spin more, and spin bigger gears, allowing you to go faster.