I'm sure the 21-speed is the right way to go, and you will learn fast on the gear changing. Key thing to remember is that the instructions you give from the left and the right hands move things in the opposite way. So your right hand twists or clicks up to change the gear up (and make it harder to ride), and twists or clicks down to change it down (and make it easier). But your left hand (which controls the front chain rings) clicks up to make it easier to ride, and down to make it harder. Once you've persuaded your brain to accept that idea, it's easy.
Of course there will always be times when you change up when you meant to change down, usually when you are going up the steepest part of the hill, and you really need a lower gear. That's when you get off in a hurry and push the rest of the way.
Have fun.
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Zero gallons to the mile