All of these require fairly decent construction (especially for figure) and good shape understanding. Latter two specifically you'd want to learn about composition, using a grid, and painting environments. Like others have mentioned definitely check out fundamentals, I'd specifically recommend Proko's Figure Drawing course to start, I think this would help a lot. Really make sure you've got good construction. Learn about perspective. Give yourself guidelines to help you make sure things are lined up. If your drawing program has a symmetry option you can use it for front-facing heads and bodies. Krenz Cushart is a good artist to look at, just google 'Krenz Cushart sketch', you will see he has a lot of anime styled art with good construction in mind. Also focus on the design decisions made: the shapes your favorite artists use, the proportions and how they differ from their more realistic counterparts, all of that. The underlying construction (which is just perspective and proportions primarily) will be #1 but still have to make sure you can make appealing shapes. Try to think about how these shapes would turn in space. Barring the last two pictures (the environment ones) you could focus on construction, then good line work, followed by flat colors, then use a shadow layer (you can use multiply that's a good layer type) thinking about basic lighting for a cel-shaded look. If you want to add environments in (makes it harder; I say just focus on the figures to start) then study composition (Framed Ink is a good book) and painting environments. TL;DR- Focus on figure drawing construction and study the shapes of your favorite artists. Check out Proko's Figure Drawing course and Krenz Cushart's sketches. P.S. Studying individual face features and hands will go a long way as well. Don't just study anime drawings! You can study from real life too; I drew a lot of pictures of old man eyes from Google and it helped improve my understand of drawing eyes. And I wouldn't worry too much about the brush you use right now. Hardround or a small Softround is fine.