Whoa. That's some pretty disturbing colors.
You need to choose a better color first. Open your MS Paint, click twice in the color you want to change in the palette window and personalize your own color. Don't use any eyesore colors (too bright and saturated) or else your sprite won't be pleasant to us. After choosing a mid-tone, make another color, a bit darker than the first. Make sure this color has a bit of hue-shift (make the color a bit bluer). Then, create a lighter version of the mid-tone color (make it a bit yellowy). You can create more colors using these steps, but I recommend you to use only 2 dark colors, 1 mid-tone color and 2 lighter ones.
EDIT: Forgot to say how to shade.
To shade, fix a lightsource and make the parts that are away from your lightsource darker (i.e, if your lightsource is at the left, your shadowed part should be at the right). If you're going to use colored outlining, shade it as a part of the sprite; if you're using a totally black outline, you can leave it as it is.
You need to know what you're shading, too. The way you've shaded your sprite makes him look like paper, and that's not good (unless it's Paper Mario style, whatever
). Make the shadows curvy so they give the 3D feel more (only at curvy parts (Body, arms, head etc); having curvy shading on a block is weird).
Also, another problem is the outlining. Don't ever mix 2x pixels with 1x pixels. It makes your sprite look poor and resized.
It could be a bit more smooth in some parts, too (shoes, clothes).
It's a hard path, but keep trying
Practicing makes you better.
You need to choose a better color first. Open your MS Paint, click twice in the color you want to change in the palette window and personalize your own color. Don't use any eyesore colors (too bright and saturated) or else your sprite won't be pleasant to us. After choosing a mid-tone, make another color, a bit darker than the first. Make sure this color has a bit of hue-shift (make the color a bit bluer). Then, create a lighter version of the mid-tone color (make it a bit yellowy). You can create more colors using these steps, but I recommend you to use only 2 dark colors, 1 mid-tone color and 2 lighter ones.
EDIT: Forgot to say how to shade.
To shade, fix a lightsource and make the parts that are away from your lightsource darker (i.e, if your lightsource is at the left, your shadowed part should be at the right). If you're going to use colored outlining, shade it as a part of the sprite; if you're using a totally black outline, you can leave it as it is.
You need to know what you're shading, too. The way you've shaded your sprite makes him look like paper, and that's not good (unless it's Paper Mario style, whatever
). Make the shadows curvy so they give the 3D feel more (only at curvy parts (Body, arms, head etc); having curvy shading on a block is weird).Also, another problem is the outlining. Don't ever mix 2x pixels with 1x pixels. It makes your sprite look poor and resized.
It could be a bit more smooth in some parts, too (shoes, clothes).
It's a hard path, but keep trying
Practicing makes you better.