Start with Sheldon Brown. To call a bike a "x" speed (where x= 10, 15, 18, 21, 27 or 30 with all kinds in between) is a marketing ploy. Most bikes only have a few nonduplicated gears. For a 10 spd you are looking at only about 6 or 7 gears that are really useful. The rest are possible but are usually the same as other gears or in combinations that don't do your bike any good mechanically.

Don't think of your bike like you think of a car. You don't have to start in 1 and shift progressively to 10 (or whatever value "x" is). Go with comfort and feel. I usually talk about gears in terms of high or low range for the front and then the rear cluster gears. For example, when I start on flat ground, I'll start in high range/middle of the rear cluster (usually the 3rd or 4th gear). If I have to climb a small hill, I'll shift down one gear in the back and go up the hill. If the hill gets steeper, I'll shift to the low range in the front and then adjust the rear to find a comfortable gear. If the hill gets really steep, I have a 3rd gear in the front and will shift even lower. That's all there really is to it. On the downhill side, I do just the opposite, gearing up until I reach a gear that is comfortable.

Don't "just put it in one gear and leave it there because it's too complicated". Shift often. Find a gear that pedals comfortably and lets you go as fast as you want.