Ok, first of all, if you don't know what it is, chances are that it's not even close to a shape. Don't make "quick things" that defy geometry. If you don't understand basic geometry, take a piece a paper and pencil and practice drawing by recreating simple shapes. Do
this first before you try doing anything else. Get the grasp down, then try pixel art.
Secondly, I looked at your shading, it's just diagonal lines moving from left to right. Ok, glad you know where the light is coming from (top left according to your image), but let's do something new.
Picture below shows
A) How you're shading your image (follow the white lines). There wasn't any thinking involved concerning the shape. Take your time if you have to. There is no rush, the better you can do this slowly, the quicker you will become later on.
B) Here's an ugly yellow sphere and the very basic demonstrating of the lightings shape of it. I used these colors to show an example in poor shading. Look how everything is shaded (follow the black lines) in a ok spherical shape, but you can hardly tell due to the colors conflicting in lightness (aka value or luminosity on your graphics editor) and crazy saturation. If the light value is the same for each shade, decrease it on each, creating a bigger space between value (just don't get carried away). For example, if I have a red light value of 80, I might decrease it by 10 (or more if I have to) for the next shade. Now they have a space of 10, because it will be an 80 red lighting and a 70 red lighting. You can apply a similar effect to saturation (although it may take more playing around to get the feel you want).
C) Now look at this cylindrical shape. My light source is in the top left, obviously. It doesn't have a great angle, but that's ok, we're trying to understand how shading creates shape. Follow the black lines, as it selects each shaded piece as it gets darker (the first one is the darkest shade in mine, but I was trying to display
design of the cylinder). Note that I use only 4 shades (those squares next to the shapes) for both the sphere and the cylinder. Try not to go over this number for now to keep things simple in the learning process.
D) You are probably wondering what that ugly cylinder with a red 'x' through it is. Well, from what you designed, I believe you were trying to create a cylinder (correct me if I am wrong) and I wanted to point out that creating two ends to a cylinder like this is impossible. Like I first said in this post, go draw some shapes on a piece of paper if you are having trouble with design.
Now this time, try recreating what I did with the cylinder by looking (don't copy paste) at how I designed it. After that, try creating a shape on your own (cube, sphere, ...).
