Knowing when and how to change the gears on your bike is not one of those things that is immediately intuitive to most people. It seems like it should be simple to do, but somehow it ends up more complicated than that and many riders new to a geared bike feel frustration the first few times as they invariably shift into a much harder (or easier) gear than the one they really wanted.
The actual shifting of gears, clicking from one to another is not difficult. It's just a matter of getting the feel for going up or down in the range of gears, and the good news is that being able to shift smoothly is about 80% practice and only about 20% understanding what's happening. In no time at all, you'll be shifting like a pro, changing gears smoothly without even thinking about it.