Hi Afflict,
I went to do a basic paintover but thought that what I wanted to explain would be too difficult to see, so I did a value study instead.
The character has changed somewhat in appearance, but I think the pose is similar enough that you'll be able to understand what I'm talking about. I tried to stay as close to your drawing as I could.

One of the things that's difficult about this image specifically is that the brim of the hat should cast a shadow directly over the eyes, which in a portrait is usually where you want your detail.
For this image, I started with a basic sketch. I then went directly over the sketch with what I thought would be my average value, or the color that's neither brightly lit nor in shadow.
I picked that color, went darker, and painted in the "shadow side" of the face, very roughly. Then I picked the average value, went lighter, and painted in the light side.
I did this again with a "highlight" value and a "deep shadow" value. The study took about an hour, but I'll admit that I started over during that time.
I don't recommend working from white and then adding color and shadow. Instead, pick a color and fill the whole canvas with it, then bring up your highlights and push back your shadow areas. You'll notice that there is no white (nor black) in this image anywhere; if I were to add it, it would seem incredibly bright/dark right away. This is a useful tool, but wasn't necessary.
Starting with a "mid value" can be more difficult in color, but experiment!
I wanted to point out a few more things.
Cast shadows are really important in an image, and you can see them here from the brim of the hat, the nose, and the upper lip, not to mention the eyelids. The thing to remember about cast shadows is that the edge of them is usually hard/sharp. Take a look at the cast shadow from the nose on the area above the lip -- it is very clear that the nose is casting a shadow onto the lip, which helps the nose "read."
Compare this with the cheek areas, the shading within the nose, and the shading around the ear. There are softer transitions here, which suggest curvature, rather than one shape being in front of another. Think of a sphere sitting on the ground -- on the sphere itself, the value changes are subtle, but the shadow that the sphere casts looks like a oval on the ground. (This is exaggerated slightly, but you get the idea, I hope!)
Finally, in my other post I mentioned eyelids... this is because we see more of them than the eyes themsleves. Eyelids are really difficult, and I didn't do them much justice here, though they are mostly in shadow. However, the bottom eyelids catch a bit of light bouncing off of the cheek, as the top of the cheek angles upwards. If you think of the eye as sitting in a socket, with eyelids wrapping around the eyeball, that might help in deciding where to draw them, and how to shade them. On the viewer right eyelid, I hope you can see that it curves around the eyeball -- even though the eye itself is shaded flat, it should read as curved (somewhat).
Anyway, I hope this has helped you somewhat!
I have a LONG, LONG way to go myself, but if you have any questions, maybe I can answer them.
Good luck!
I went to do a basic paintover but thought that what I wanted to explain would be too difficult to see, so I did a value study instead.
The character has changed somewhat in appearance, but I think the pose is similar enough that you'll be able to understand what I'm talking about. I tried to stay as close to your drawing as I could.

One of the things that's difficult about this image specifically is that the brim of the hat should cast a shadow directly over the eyes, which in a portrait is usually where you want your detail.
For this image, I started with a basic sketch. I then went directly over the sketch with what I thought would be my average value, or the color that's neither brightly lit nor in shadow.
I picked that color, went darker, and painted in the "shadow side" of the face, very roughly. Then I picked the average value, went lighter, and painted in the light side.
I did this again with a "highlight" value and a "deep shadow" value. The study took about an hour, but I'll admit that I started over during that time.
I don't recommend working from white and then adding color and shadow. Instead, pick a color and fill the whole canvas with it, then bring up your highlights and push back your shadow areas. You'll notice that there is no white (nor black) in this image anywhere; if I were to add it, it would seem incredibly bright/dark right away. This is a useful tool, but wasn't necessary.
Starting with a "mid value" can be more difficult in color, but experiment!

I wanted to point out a few more things.
Cast shadows are really important in an image, and you can see them here from the brim of the hat, the nose, and the upper lip, not to mention the eyelids. The thing to remember about cast shadows is that the edge of them is usually hard/sharp. Take a look at the cast shadow from the nose on the area above the lip -- it is very clear that the nose is casting a shadow onto the lip, which helps the nose "read."
Compare this with the cheek areas, the shading within the nose, and the shading around the ear. There are softer transitions here, which suggest curvature, rather than one shape being in front of another. Think of a sphere sitting on the ground -- on the sphere itself, the value changes are subtle, but the shadow that the sphere casts looks like a oval on the ground. (This is exaggerated slightly, but you get the idea, I hope!)
Finally, in my other post I mentioned eyelids... this is because we see more of them than the eyes themsleves. Eyelids are really difficult, and I didn't do them much justice here, though they are mostly in shadow. However, the bottom eyelids catch a bit of light bouncing off of the cheek, as the top of the cheek angles upwards. If you think of the eye as sitting in a socket, with eyelids wrapping around the eyeball, that might help in deciding where to draw them, and how to shade them. On the viewer right eyelid, I hope you can see that it curves around the eyeball -- even though the eye itself is shaded flat, it should read as curved (somewhat).
Anyway, I hope this has helped you somewhat!

Good luck!
