There's a ton of tutorials online on shading anime characters, try watching those and speedpaints. Anime shading is mainly cel shading, then further enhanced with soft brushes, gradients and layer effects.
OK, I made a color map for you. Since you didn't indicate otherwise, I assumed you wanted to continue cel-shading.
I'll walk you through the process.
Goal: To use colors and shading to indicate:
For this, right now, we're not going to worry about bounce and ambient lighting. Those are things to think about later on, not now.
1. STEP 1 - determine light source.
Your first (pre-edit) comment said the light was coming "from the face." Since that's what I had read before I got to work on it, I assume you wanted a light source between the face and the snake. I indicated that with a yellow blob.
I also assumed a warm light temperature.
Your new comment/edit makes it sound like you wanted the light source up and center, but I only saw that once I'd already finished. So let's go with the first one for this explanation, since a different light source would place lights and shadows in different places.
2. STEP 2 - color palette
Your original image shows basically flat-shading (the shadows are very, very minimal) except for on the hair. For cel-shading, try to use at least three colors:
1. base color
2. shadow color
3. highlights color
Here, it's important to take into account the temperature of the light source. To keep things SUPER simple, a warm light casts cool shadows (this depends on a lot of factors, but let's keep it simple and go with that right now) and a cool light casts warm shadows. (This is a generalization I don't particularly like, but it will keep things simple for us.)
I expanded your original color palette like this:
HAIR - kept the base and shadow color, added highlight color and accent color. Your shadow color could actually go a little colder (more towards blue, but very subtly), but I kept it like you had it.
COLLAR - kept base color, added shadow color
SKIN - kept base color, replaced original shadow color with 2 shadows that tend more to purple-ish
ARMOR LIGHT PARTS - kept base color, added shadow and highlight colors
ARMOR DARK PARTS - kept base color, added shadow and highlight colors
CHAIN MAIL - kept base color, added shadow color only*
SNAKE - kept base colors, added shadow and highlight colors
*This looked like some sort of wire webbing against fabric; if so, it wouldn't reflect light (as a whole) that much, so I skipped the highlight color.
Alright, now, let's goooo!!!
3. STEP 3 - HIGHLIGHTS
I've marked areas to be highlighted with this light blue. I've marked the corresponding color in the palette with the same color.
The area markings I've done for this walkthrough are rough and jagged bc I was trying to work quickly; you make them as smooth as you need to.
OK, so now we look at the light source again, so our yellow blob is back.
Take the color indicated by the light blue marks and highlight the areas of your drawing that are directly hit by the light source.
Take shape into account (see how I indicated part of the cheek here).
4. STEP 4- 1ST SHADOWS ON SKIN
In anime, skin usually has more than one shadow color going at once, so that's why we have two shadow colors here. This is the lighter of the two, indicated by this blue-ish color.
Take the first shadow color and shade the skin in the areas indicated, taking into account shape (see: nose) and cast shadows (shadows cast by the hair).
5. STEP 5 - 2ND SHADOWS ON SKIN
See color and areas indicated by this deeper blue-ish color.
The chin/jaw will cast a darker shadow, so we're using this color here, as well as on the back of the neck (area farthest away from the light source). You had originally done the shadow cast by the jaw in black; I replaced that with this darker skin color to make the shading more harmonious/cohesive.
6. STEP 6 - HAIR ACCENT SHADOW
This adds interest. It's optional, but looks cool.
I indicated the color and the areas to use it on, there, in blue. It's purple-ish because of that warm light / cool shadows thing.
7. STEP 7 - SHADOWS ON EVERYTHING ELSE
Now that we've taken care of the skin and hair, let's look at everything else.
The highlights were already indicated in Step 3.
I've indicated the color in the palettes and the areas to apply it to with this teal color.
On the snake, I used that dark red bc the teal was too much like the snake's color.
When applying shadows, once again take into account SHAPE as well as light source placement. This is what Cicakkia meant by needing to know the form of your character; you need to understand the "topography" of the thing you're shading.
Also be mindful of shadows cast by other parts of the drawing (like the plates of the armor, and the shadow cast by the snake). I've indicated the shadow cast by the snake in red, but you'd use the shadow color of the surface it's cast on (so, the armor shadow colors)
8. STEP 8 - ARMOR SUPER HIGHLIGHTS
Metals are extremely reflective, so that the lightest parts are basically white.
Add thin white lines to the armor following the areas indicated in red here:
OK, that took longer than I anticipated I hope it helps. If you have any questions, let me know.
Do you have any attempts you can show us so we can better guide you?
Also, "shade to make it look like an anime character" can go many ways... What kind of "anime" type of art do you have in mind as a goal for this?
But what Cicakkia said is true - By understanding how to use shading to represent volume and lighting, you'll be able to do this. That's the underlying skill. It will take a while to perfect this, but if you post a picture of where you're at right now, as well as describe (or provide visuals) of what you're trying to achieve, we can give you more targeted advice.
Also, do you want the light source to be coming from the top, or the top-right? (based on the shadows on the hair) I can give you some "red-line" (visual) suggestions on that pic, but I'd kind of need to know the light source for sure first