You are over thinking this WAAAAY too much! My kid has it all figured out...if it gets too hard to pedal, shift...if it gets too easy, shift. It's as simple as that. But someone here will find a way to over complicate this and start putting in heart rate, cadence, angle of the hill, speed of approach, energy expenditure, calories burned, ass pain, the phase of the moon, the tides, etc., and one will find a way to put in an equation and wow you with their wealth of knowledge.

As a beginner, you really need to just go out there and figure out a way that will work for you. Ride and HTFU!