The light source is handled very well as it's very easy to tell where it could be. Part of the trickiness of shading can be the base colors. For example, stark white or pure black are especially tricky as they can't go any farther in a direction for values. Using a very light grey with maybe a little bit of hue is great substitute for white. In addition the the shirt is barely shaded to begin, making it feel very flat in comparison to the rest of the character. Clothes tend to have a bit of a learning curve with wrinkles and textures. References can help a ton with that.

Speaking of clothes, adding in wrinkles can help out a lot. Generally it'd be around joints. References can help a lot with this as well. I find the site at the bottom very helpful for practice thanks to the randomness if it's something you want to work one. (Also poses but still) Pinterest also has a ton of options for it as well.

Hope this helps!

https://quickposes.com/en/gestures/random

I know this doesn't really seem like good advice, but: Do your best! As long as it makes you happy and you're doing something that you love & enjoy, it'll be awesome! Be proud of your art and yourself!