Okay, here's the edit I told you I made yesterday :3



The big corner in your hair, somehow, I managed to NOT correct it even though it was that particular detail what prompted me to do the edit. At least it's marked with a red circle in the original (to the left) Most of the corrections are technical and hard to notice, but still:

-The way your hair falls on / interfaces with your forehead is too regular. There are several rows of different colors, but exactly the same size, if I'm not mistaken, that's banding.

- your face had an outline of a color that's too light, I can understand that it's an attempt at lighting, but unless you have a really strong light source, it shouldn't be like that (maybe someone more knowledgeable can explain you clearly *why*, I can only tell you *what*)

- Your forearm had some chunky shapes that I refined a bit, and also some banding.

-The wrinkles on your forehead are cool, but I added some intermediate colors at their ends and start to make them less abrupt. (antialias)

-I softened and refined the shape of the ¿sinew? in your neck. You know, that wire that connects muscles. Or maybe it's a muscle. I don't know, I'm no anatomist.

-Apparently I put a white pixel by mistake in one of your brows. My bad! 

General advice:

1 - Use a indexed color format (256 colors tops) that'll give you better control over the palette you're using, and will teach you to think in discrete colors, which in turn will teach you to MAKE the colors you want (kinda like mixing colors with traditional paint, but totally different at the same time)

2 - I don't know what program you made this with, but it must be made in shotofop or something similar because it's in some color format other than indexed/256. I personally recommend GrafX2, it's free, it's widely used in the forum AFAIK, and now it even boasts animation support. It's quicker and less cumbersome that other graphic editors (of the "big", general purpose, virtual paints or photo editing kinds), weighs a lot less, and has a lot of functionalities *especially tailored* towards pixel art.

No, they don't pay me any amount of money, but FIRST I knew about GrafX2, some eight years ago (back in the day when it was MS-DOS only!) and THEN, after using it, learned what this "pixel art" weird artistic discipline was.

3 - You need to reduce color count: you use several different colors that read same-y to the naked eye.

4 - The t-shirt is a total chaos I daren't touch sorry.

Besides all that, you seem to have a fair grasp on representing the human form, AND realistic shapes, and you look pretty permeable to useful outside information, so just keep at it, try to understand the already discovered techniques and learn from other artists. If you are intent on improving, I'm sure we'll see more and better pieces from you and you will develop your own style (or even styles in plural!)

Getting any knowledge about art in general is also very helpful, even if sometimes more indirectly. I do find that hard to do myself, but I still know it's a good idea: Any knowledge you manage to gather on anatomy, composition, color theory, or whatever, WILL eventually pay out. You might come up with good stuff by yourself anyways, but *knowing more* always helps, never hinders.