The aim of gearing is to match your pedalling power output to the riding conditions.
Your power output is fixed, when your riding along, you can't press the accelerator.
At any fixed power output, you can vary your pedalling cadence. Spinning faster in a lower gear works your heart and lungs, pedalling more slowly but harder in a higher gear works your legs. Most newbies pedal too slowly for efficiency, you should aim for about 80revs/min in a gear that allows you to pedal freely.
So, you are cruising along at your fixed power, at 80 revs/min. I don't know how fast you will be riding and I don't care and neither should you.
You hit an uphill. You change down to a larger rear cog or a smaller front chanring or both. Now you are pedalling along at your fixed power output at 80revs/min but riding more slowly.
You hit a headwind, you gear down a bit more, pedalling the same, and go more slowly.
You turn around and get a tailwind. You gear up, to a larger chainring and/or smaller rear cog. Using the same power output and pedalling, you now go a lot faster.
You should try to avoid gears that cross the chain to extremes (eg small chainring to big cog, or even the next 2 smaller cogs the the inverse with the big chainring).
You should note the overlap between your 3 chainrings. You can get exactly the same gear using 2 different combinations and it doesnt matter much which you use.
Shifting from one chainring to another is a big step in gear, too much for comfort. You should get into a habit of changing chainrings and rear cogs at the same time to make a small step in gear, ie middle to small chainring so you move to a smaller cog at the rear to give a gear just a bit lower than before.