I'm not an expert, but here are two tips that helped me improve a lot:
1) I see the sprite that you linked is black and white, but when you want to try color, continue working with a limited palette (maybe 16-64 colors). This is much easier than trying to pick from several million colors. Actually creating a palette also takes experience, though, so either start VERY small (like 4 colors), or find an existing palette that appeals to you, like EGA or something from https://lospec.com/palette-list.
2) Study pixel art that you like. Zoom in, examine pixel by pixel, look at proportions and angles and colors and outlines, figure out the techniques that make it look the way it does. Then try to reproduce those effects in your own art. Even a single pixel can make a big difference in how the image turns out, so studying their placements and patterns can help you understand how to get the results you want.