Hands: Really tough for a lot of people, I've found it useful to try and make the pose I wanted with my own hand and try and use that for reference though.
Kids/Teens: not much different from adults (especially teens are basically full grown, a lot of them, and thus just have a slightly younger look about them). I guess the main thing with kids though is that it's common for like really little kids to have a bigger head to body ratio than that of people, say, middle school and up? The amount of exaggeration placed on that varies style to style though.
Rain: tutorial above looks pretty good, don't think I have much more than that already says lol
Backgrounds: too wide a subject to tackle all at once. The underlying principal that applies to all backgrounds, though, is perspective. So looking up explanations and tips for that will help as a starting point! As for drawing different types of backgrounds, i can't emphasize using reference enough. For me, at least, the thing I identified that made a lot of my backgrounds look really subpar was just that they were lacking too much detail and texture. But it was to hard for me to figure out how to portray those things on my own, so I had to start looking up examples to supplement my backgrounds and they started shaping up better.
