Here's a really quick sketch of what I would have in mind when spriting a tree along some notes:
- Use reference
- Always establish a clear direction of the lightsource.
- Avoid outlining inside the sprite.
- Group shapes, you don't want to draw every single leaf, just draw some big shapes that resemble a cluster of leafs.
- Add some variation to the tree trunk by adding more interest to it, a little texture, some branches, some roots, a hole, etc.
- Maybe it's better to add a small patch of grass on the bottom of the tree to ground it a bit, interactions between stuff is a strong point to sell believability.
(By the way, this forum has a built in zoom function, so always post your sprites at 100%, if you need to zoom some image just click on it)

You can also check some tutorials by Saint11 (
https://saint11.org/blog/pixel-art-tutorials/ ) that are great for beginners, here's one on trees:
