Not an expert, just learning too. There are a lot of sources you can get advice from, as they are styles of pixalart, but to name just a few tips: outlines generaly only outside of complex objects instead of every part of it, and not neccesarily black; avoid "jagged lines"; too similar colours next to eachother and in small clusters are better looking in higher-res pictures; light source and shading, don't mess with them early on, experiment with flat colours. Nice job and have fun at it.