I really want to improve my skills!

[Gors]

first than learning a 'high-level' technique such as anti-aliasing, you gotta learn the very basics first.

You apparently have some basics already, as shown in those sprites. I'll comment only about the second sprite you made, as it's one of your recent ones with apparent problems.

First of all, your lineart, as Curly said, is jagged. The lines are really straight when it should be really smooth, and there are some broken parts in the line that throws me off. The gray color you used for outline on the robes/boots are lighter than it should be (the outline color shouldn't be lighter than the shade inside it) and you don't need that many colors for a sprite that size. Pixelart doesn't get better if you use a lot of colors; it gets better if you use the colors in an intelligent way, lowering the numbers of colors as many as possible. This would be what I consider basic problems.

After fixing those problems, other changes could be made, such as: choose more contrasting colors for the light gray you used for the scythe, because metallic objects suffer higher contrast between its shades than other materials such as flesh, cloth or plastic. Also hue-shifting your colors would add a nice touch here. Readability is also everything in an art media like pixelart, so be sure to represent the character better than trying to add unecessary details. This will make your sprite improve tenfold.