I struggle when trying to shade a character. I use top left as a light source. The directions of the three sprites are given in the following image:
Can someone help me?
I would highly recommend that you consider studying how shading works in the real world and draw from observation in a sketch book as opposed to diving right into it with pixel art.
Right now it just looks like you're applying different values of the same color on different parts of the body and it still looks flat. Things you have to consider when it comes to shading is the shape of an object, where the light falls on it, and a couple of other technical considerations as well.
You can probably find a couple of basic tutorials on this subject on google, deviant art or youtube for more information if you're really invested in practicing this. I know it may seem a little odd to suggest working in a different medium of art when this is a pixel art board, but from my perspective I find that it helps tremendously to work in different forms of art and learn from making several bad works. I'm speaking from at least 6 years experience of studying art and animation in college and plus a few years of making unprofessional crap art to back that suggestion as well.
Don't forget the importance of color contrasts when shading pixel art. This tutorial assumes you already understand color theory and such, but it's a decent crash course in pixel art basics
https://forums.terraria.org/index.php?th...ing.18031/
The other way I'm learning is by paying close attention to 2D games that I play. Analyze the sprites, animations and differences between style in different games. Figure out what style appeals to you, personally I started with sprite edits to learn constrictions of different games/systems for practice and started making more original work. I'm still not really confident in my work yet but hopefully that helps.