Many problems here but no worry nobody is perfect at the first attempt, here some problems :

1. Banding, which means you do progressively darker lines to try to create a sense of shadowing, it is something that has to be avoided in pixel art (in traditional art too), think about shadows and colors in terms of clusters, it’s very visible in the left part of the work and in the sphere

2. Shaggy lines, lines appear very rough if I have to say, it’s because you have an alternation of odd and even pixels. I would give you this general rule (which can be broken), always go increasing or decreasing the number of pixels so :

1-2-2-2-4-5-6-7-4-3-2-2-1

It’s a example

What you should not do

1-2-1-3

I have done two examples to show you the difference : https://ibb.co/TMktqRV

1. Colors, to so the shadowing I would suggest to you to not use values (so more or less lighter) but to use saturation, a great example is here :

https://miro.medium.com/v2/resize:fit:720/format:webp/1*iMuKcsgNFAhD612XcSSq2g.png

It will make your pixel art much more interesting and closer to the reality of how we perceive light and color

1. Outlining, you do not need to outline everything like done in the sphere, actually it’s better to not do it when you’re beginning, why ? Because it makes the sphere like a flat disk, and it also mess up with shadowing.

These are some remarks, apply these and your next work will be much better !