Going back to the gears ... not sure how many you have on that bike ... but for example if there were 3 on the front, and 7 on the back then you don't get a straight forward sequence such as ...
Front 1, Back 1,2,3,4,5,6,7 and then Front 2, Back 1,2,3,4,5 6,7, then Front 3, Back 1,2,3,4,5,6,7
Instead they overlap so that pedaling in Front 2, Back 1 might be the same as using Front 1, Back 3..... so you wouldn't actually have 21 different gears.
The easiest way to think of it is that the Front gears are big jumps in gears (counting in tens), and the back ones are fine adjustment (counting in ones)...so if you stick to the middle gear and get to know the rear ones first ... and if its hard going up one of those hills then swap down to 1 on the front.
Good luck and feel free to ask more questions .... they're just the sort of questions I was asking when I started.