Hello! I'm not too experienced myself, but here are a couple of things I noticed that could use some fine-tuning:
Be aware of your outlines and try to be consistent with them. To make things look more streamlined, try to keep the widths of the stroke the same and also try to keep all of them the same color (or you could try having the outlines be a slightly different shade than its fill). The guy in the first image has really thin arms that are smaller than the other outlines.
Try to limit your color palette. Having too many colors can muddy up a picture. Also you want to try to make sure the colors you choose are a similar intensity. For example, try to pick only colors that are bold, or only pastels, or only muted. The rocket, to me, has too many unrelated colors. However, the witch looks pretty good as well as the guy smoking.
On top of that, also make sure to choose your colors carefully. The yellow of the smoke on your rocket is kind of green and doesn't fit well with the other colors.
Of course there are ALWAYS exceptions to the rules; the trick is just knowing when it's okay to break them! Sometimes it's good to start out following the "rules" and as you learn, you can take more and more steps outside of the box.
Overall I think it's a good start! And you mentioned being worried about your shadows, but I think they actually look pretty good so far. As for art classes, there are plenty of free resources on youtube! Just keep working at it and you'll get better everyday.