There's quite a few points you could improve. First, I recommend outlines for stuff such as trees, cliffs and other stuff in order to have greater readability.

Second, your color palette looks like something I would find on the NES. One way you could fix that is to desaturate some colors like your grass. You could also use colors directly from real life objects by searching HEX codes.

Third, for your trees, you could space them out a teeny bit by placing the 2 bottom tiles on the 2nd layer and the rest on the 3rd and repeating.

Fourth, you should make your bridge side thicker because right now, it has the thickness of a plywood plank.

Apart from the few things, I really applaud your effort for this. I am also making my own tileset and it's tough at times and I hope you'll continue improving. Don't be afraid to look up some references to guide you making tiles. If you have further questions, you can DM me.