Hello there K_B. Welcome to the forum, and pixel art . Let's go through this step by step:

1. Line art - youve got some jaggedness on his legs and shoulders. Also, I probably would get rid of the black outlines and replace them with coloured ones, but leave it for now if you wish. Here's 1 tutorial on clean lines, but I'd recommend seeking out more, try the support section of the forum for links to more.
2. Colours - the colours you've chosen are too saturated and don't work together well. ((incidently to anyone out there, are there any good tutorials which address hue&saturation shifting?)). One of your greens and at least one of your blues are almost invisible, so they are unnecessary. Trying to work with the least number of colours possible will make work more managable. I think you can get away with:
- 2 skin tones (which are was too saturated (meaning they need to be greyer))
- 2, maybe 3 blues
- 1, maybe 2 greens
- 2 greys
3. Shading - What you've got now is referred to as pillow shading, which just won't do! There's no doubt LOTs of tutorials on shading out there, not just in regards to pixel art. Seeking out any tutorials on shading will benefit you. Here is one on shading that I find particularly excellent, but I would encourage you to seek out others.

Um, ok that's all I've got for now...just woke up so there's probably things I haven't thought of.

Oh, who can go past Tsugumo's Tutorials? Truly classic stuff, if for nothing else you should check it out just for the lore, but it's also an engaging investigation of pixel art, hell, it's where I started, now that I think about it...