This is a sketch I'm not too happy about. Please give some constructive critic (which you never get on DA).

If you want constructive criticism, ask for it. There's a club for people who want to be criticized, too.
Anyway. As far as I can tell, her eyes are a bit far up her head/her forehead is a little small. Her face looks masculine, too, which might be connected to the mouth being slightly misplaced.
I can't really figure out the hood...... or whatever that is on her head. I mean, it extends to the tip of her rod? And it suddenly turns stiff and straight around somewhere around her collarbone region?
Furthermore, the shape of her body is strange, but it's hard to put, why. Her shoulder which can be seen on the left seems to be too high up in relation to her breasts, which rival the size of her head, and which either lie too low, or just look like it because her chest is too big. Maybe her neck is too short, too.
And finally, her waist seems to become really thin. Of course, the picture doesn't continue there, but I'm guessing that the two lines are still her body.

...and it just occured to me that the hood probably extends into a cloak. That isn't made clear, though. Look for references.

Other than that, nice hands and details.

Her mouth looks a bit weird in my opinion. Otherwise, the things Fawriel said, the rest is fine.

You still did a way better job than I ever could.

Like Faw already mentioned, the six-inch waist doesn't really work with her breasts the size of her head.

Since the other two have already torn your anatomical mistakes into tiny, bite sized pieces, I'll critique a different aspect: action, color, and focal point.

Now, action is important. In most pictures you want to show something happening and have lots of details support this action, much like you would support a thesis in an essay. The first thing that becomes apparent in the picture is that she is using her staff thing to shoot magic ice stuff into the lower right corner of the page. Maybe she's attacking something. Or maybe she's cooling some soup. For further hints to what is going on, we look at smaller details, like her face. She seems to be giving an evil, confident stare at whatever she's icing. So maybe she's attacking someone tiny and weak. Or that soup burned the top of her mouth and she is getting revenge on it. Lastly, we'd look at things like her cloak and pose. It's curved to the right as if she has just turned around. From her rigid pose, it doesn't really look as though she has turned around quickly, so whatever she was icing probably wasn't all that important. Therefore, I conclude that she has just turned around and is now using ice magic on something that isn't all that relevant.
I can't really say how to fix this up, becuase I don't know how it was intended to come across. Twist her body a little more if she was meant to be turning. If she's supposed to look powerful, try adding perspective so we're looking up at her. If she's supposed to look scary and intimidating, change the pose so she's attacking the viewer. A good thing to learn is that you should draw weapons with very straight or smooth lines. Floppy looking weapons just aren't good looking.

I'm not sure if the choice of colors was intentional, but the blue, white, and black work for this image theme-wise. However, they throw the focal point off. When you first look at the image, her hands and breasts draw your attention, as they are in the middle of the page and the color of her hands contrasts the white background. Given the picture, you probably want to have the focal point be her face or her weapon. Sometimes, it can also help to add a background which can move focus towards where you want it.

There is a lot more to art than anatomy. Pictures convey feelings, stories, and ideas. Take control of the viewer's thoughts for a moment and hide a simple story or idea within even the most basic sketches.