Good to see a budding pixel artist. Critiques:
1. Too much white space around the bananna, too little extra space around the profile portrait.
2. Think in a realistic sense, use proportions and scale (but don't do lose your style).
3. An idea, maybe dither some of the browns to make a more natural hairline, it's a bit like it's brown matter, but not really hair. See what the others say about the hair, then work on it.
4. Since it's quite small, up the contrast a bit.
There are more so I made a small edit; the white was added to show you what errors the background was keeping you from seeing. I fixed quite a bit, so zoom in and compare:
->
Other than that, really good start so far for some of your first pieces of art.