I'm pretty new as well but I thought i could offer some suggestions. I think that the overall style needs to stay more consistent. For example his legs look very blocky, and the rest of the body is much more curved. He has really detailed 3 tone shading under the hood but not much anywhere else.  

I think the body shape looks good. I'm not sure about what you want the style to be but the head looks too circlular to me. The eyes have a nice lost boy feel to them idk why. The shadows could make more sense, so shadows at the bottom of his head and under the jacket. I think overall everything is a good start except for the backpack. In my opinion the backpack needs to be completely redone. THe straps are very awkward and the thing itself. I'm not sure what the dark parts are supposed to represent.

Nice job and good luck :)

Have a look at this, I learned quite a lot with them:

https://makegames.tumblr.com/post/42648699708/pixel-art-tutorial

https://www.phonoforest.com/2020/05/3-major-mistakes-pixel-art-beginners...

https://web.archive.org/web/20110305165124/http://kiwinuptuo.deviantart....

But the most basic way is to first get some similar references to what you want to do. It really helps to know where everything naturally goes, you can build over it with the lesson from the first tutorial.

https://www.google.com/imgres?imgurl=https%3A%2F%2Fimg.freepik.com%2Fpre...

https://www.alamy.es/foto-enmascarado-macho-hoodie-con-pistola-de-mano-h...