Hi Sen, welcome to Pixelation.

I did a quick edit on your piece to show how it can be made more readable. It was a mistake to separate the shadow from the cables, as it is a
cast shadow, and therefore physically impossible to be separate unless you want to imply that the cables are floating above the desk! I suspected they are not, and you will be happy to know that whether the intention was to remove aliasing or to imply a shadow makes little difference in this case.
However the monitor noticeably lacks a cast shadow, even though it is the tallest object on the desk. I also found that the cables were too messy, to the point that they are distracting from the whole piece. And where are they all going, exactly? I plugged them into the wall. Another issue was proportion, the mouse and monitor too small in relation to the keyboard. I enlarged them a bit, but this monitor can definitely be larger still.

As for how to improve in general, study light and shadow around you. Pencil and sketchbook, just draw whatever catches your eye in your room or around your house, find the shadows on and around objects. Study their form. It does not have to be perfect. But with just the act of practicing you will come to learn how light works.