The Student's Brew

Hey there!
First off, good job on asking for critique. I know you've been trying to improve your skills lately and getting feedback is a great start on knowing what you should focus on.

There are a couple of things in this piece that I think you can improve on, and I think the most important one is the figure. I know you're primarily an anthro/pony artist, and so humans are understandably something that you're probably not as comfortable with, but learning to do them right will help you in all areas of your work. The biggest thing in here are the eyes. They're definitely showing your pony influence, and I don't think it's in a good way; while some art styles have very large eyes (certain shoujo anime styles, certain chibi styles, etc), they still have to fit into the facial proportions in a pleasing way to be acceptable. In this case they're so large they appear crammed into the face, and it's enormously distracting from the rest of the picture; while there's a lot going on in this composition to keep the viewer moving around, it's hard to get away from those eyes. This is something that's pretty common in your work and if you plan to stick with a very large-eyed style, I'd definitely consider figuring out a more pleasing set of proportions or level of elaboration within the eye itself.
Aside from the eyes, there are a few areas of the figure to watch out for. You've got one breast drawn in with a line and no shading; typically you'd only have such a defined line in something skintight, and even then there would be a LOT of shading to back it up. More commonly, and especially in a loose garment like you've shown here, fabric doesn't really cling to the breasts in a way that defines them seperately. You'll have more of a general swell across the entire chest at that level-- it's hard to put in words, but look at any picture of a girl in an average t-shirt and you should see what I mean. In anything but the tightest clothing, breasts typically aren't individually defined.
For her sleeves, I understand the general tattered look you're going for, but they read a bit more like draping fur. Consider the fall of the fabric and that you'd probably see tatters on either side of the arm.
Her hat is also a little funky in that it's placed oddly on her head; it doesn't actually appear to be seated properly, but rather looks like it's in the process of being blown off or levitating. Hats are a pain to draw in general, but I would recommend that in the sketch phase you just draw the head before drawing the hat directly over it; it'll help you figure out where the hat's seated.

With the figure aside, I would say you should work a bit on your rounding of forms. You're starting to include backgrounds, and that's wonderful! It's very easy to fall into the trap of one-figure-on-white-space, and the fact that you're starting to draw your backgrounds early is great because it means you can learn both background and figure as you go, and keep your skill level consistent on both.
At this point, I think you need to work on making your forms believeable. I'm not entirely sure where you are with your art education in any sort of formal setting, so this may be stuff that you'll be learning soon anyways if you're taking any sort of classes, but you currently draw things in a very flat sort of fashion. The overlapping of form conveys some depth, but there's very little to suggest rounding on the bottles, the rounding of the cauldron handle, the protrusion of the lip of the counter, etc etc. Places like the book leaning against the wall is where this becomes most evident, since it's hard to tell whether it's cover-or-page side up ( > or < , if that makes sense). I know this is a fairly close-up picture and so perspective would be pretty minimal, which is letting you get away with it here, but in general it's something to keep in mind. If you are thinking of doing any studies in the near future, consider working on some perspective studies just to get yourself used to drawing the volume of objects.
On a small side note, you also tend to draw kind of phallic bottles. It's not really an art issue, but just something that might be a distraction XD

Lastly, lighting! I can see this is something you're still playing around with, and I like that you've worked with an overall atmospheric light/shadow pattern in this picture; it gives a nice effect overall and definitely speaks to the environment this scene is taking place in. You've done a nice job with that, and also by using a color to shade rather than just darkening everything.
However, it's really important to pick a light source and stick with it. You've done a fairly good job with that overall (I can tell roughly where you want your light source to be), but there are areas like the props on the counter where it seems like you've forgotten to put any shading in at all. It's also very important to consider the interaction of objects in terms of how they block the light; by this I mean that the cauldron would cast a shadow on the body of the witch, the lip of the counter would cast a shadow on the vertical surface of the counter directly underneath it, etc. You also have a few areas, like the substance in the cauldron itself, where it seems like the light source is reversed.
I'd also recommend adding some highlights when you're comfortable with them, since they can really add a nice pop to metallic objects!
Overall though, the lighting isn't bad at all! It just needs a little work and consistency, and I know that this is a very complex picture so it can get pretty overwhelming.

As a whole, I think this picture is very ambitious and it's wonderful you're working on complex images with a background as you dive into comic-making. You're definitely on the right track and the fact that you're seeking feedback and doing things like the anatomy studies you mentioned will serve you well; you've got a lot of potential and I think you can go far if you just keep practicing and learning! If you keep some of the things I've mentioned here in mind for future pictures, I think it might really help.
I can't wait to see what you come up with!