Hi Sas! What direction you take with your shadows depends on what you're trying to do, because there are all kinds of styles. But if you're wanting to make it look more realistic, I'd suggest using yourself as a model and making observations about how shadows behave. (I have a photo library of ridiculous mirror shots which I hope no one ever sees!)
First of all, your character design and your idea are quite cute! So there's definitely something there in your imagination that's worth pulling out and learning to draw the way you want it to look.
For shading, start by thinking about where your light source is, or where you want it to be. Here it looks like you've got it coming from above, but it's a bit shallow (as in, the shadows make thin stripes suggesting shallow depth), which makes your drawing look like a relief. Lit from above, the eyelids would put more shadow on the top of the eyeball, but not necessarily all the way around. The shadows currently make the most sense around the mouth, which looks realistic and where you have some indication of a cast shadow. There should also be one under the neck, and on the top part of the ears, for example. If you look in the mirror with light coming from above you will see this right away.
But also, think about the mood you are trying to create. Light coming from above looks a bit like an interrogation (think of police shows). Observe the light around you and decide which kind of lighting suits the mood you want. Maybe for the mouse this scene is an interrogation! But your giant looks friendly, so maybe not.
Also, think about how the eyes fit into the sockets and how the skin goes over them. (There's a great part of a critique in one of the online classes somewhere that talks about this very thing with eyeballs, but unfortunately I have no idea where it is!)
And finally, for the nostril, I think your placement on the nose is fine, but the very dark value draws a lot of attention and the angle of the nostril looks wrong along the bottom. (The eye gravitates towards value contrasts.) Look at your own nose in the mirror with your face at the same angle. You will see some skin along the bottom of the nostril and perhaps part of the septum inside. And probably the bottom of the shadow won't line up with the outside contour of the rest of the nose.
Sorry, that's a lot of stuff, but once you know a few things about light and shadow you will improve really fast and it's fun! If you subscribe to the videos, have a look at Jake's Light and Shadow class. I'm sure there are others, but right now they've all kind of blurred together in my mind and I can't remember what's where anymore. Just filter for fundamentals and lots of good stuff should come up.
And if you want, post successive versions and ask again. I've seen people get really good critiques that way!