In stead of telling you "to work on anatomy and shading" I'll help you a bit. ^_^
Form:
Try starting with the head, an oval that is pointed toward the chin.
Then a line for the motion (curvy for spayed hip, straight for standing straight-duh-, and triangle for bending or sitting.)
Then another line for the breastbone (not the shoulder. breastbone is more stable) about a head length under the chin.
This is the way I've been doing it for years. It may seem a little funky at first but the more you study and practice the better you get of coarse.
Shading:
Shading on the comp can be fairly simple if your not trying to be Michaelangelo or something.
1. Color. Start with a medium color: red/orange red/ brown red.
Fill in the areas you want this color in. Hair
2. Shadow. Shadow before you highlight because shadows are always under something. ^_^
Chose a darker color then your medium color: blood red/ dark brown/ black.
Chose where the shadows should go and fill in, under the curls, in the divide, around the ears, anywhere opposite of the light source.
3. Highlight. Highlight last and sparingly. Highlights are only there for the sake of some dim lights.
Chose a lighter color then your base color. This can be a lot of colors depending on what kind of light you’re in.
Depending on the intensity of light the color can be a shad or two lighter (wide) or stark white. (Narrow)
Fill in small jagged fragments/lines of lightness until you're satisfied.
Like form, practice is your friend. If you don't like the results start over or erase, and don't forget you can undo the last 3 steps in paint.
I hope you don't mind me using your pic as an example. (Or being a know-it-all, as I tend to be.)
Just keep in mind this is not the only one way to do it. I like to refer to it as the 3 tone way (or 5, or 7, or 12 depending on detail) other people do it much differently (and better usually) but this is the way I know how to do it.
Edit: Sorry I did this in PhotoShop not Paint. It's still the same concept, just not the same results.