Little and often is the best way to get fitter for cycling. Mainly its about conditioning yourself to the cycling position and toughening your butt. You dont need to do anything as extreme as training, just ride along at a breathing rate equivelent to a brisk walk.
The human body has a very narrow power range, essentially constant. Gears match your power output to the conditions. You can vary your pedalling force and speed (cadence); the gradient, surface and wind can vary as you travel. Gears enable you to select your desired pedalling force and cadence , you just have to let speed take care of itself. A good cadence to start with is about 60rpm but most experts go faster, about 80rpm.
As the drag forces get higher (steep hills, headwinds) you need to shift down to a larger rear cog.
Generally you stay in the middle chainring at the front until you run out of rear cogs then shift to a smaller chainring to get a much lower ratio of gear. You may have to to shift to a smaller cog to make a small step change in gear ratio.