First, I think it might be easier if you know what kind of tree it is. Trees vary a lot in shape and how their leaves and branches are arranged. In any case, I think it would be a good idea to find some reference.
Tree trunks are more straight than that of your tree. Think of branches as the trunk split in smaller parts. That means that branches shouldn't add up to be larger than the trunk they originated from. In your tree, the base of one of the branches alone is thicker than the middle of the tree trunk. A tree like that would easily break in half in a bit of wind.
Now, if you want something stylistic instead of realistic and want to keep the solid tree crown, I would still recommend looking at some references to get the shape right. You have the right side of the crown drooping, but the rest is not. Trees aren't usually perfectly symmetrical, but the positions of the branches and leaf clusters follow the same principle throughout the crown (same branch thickness, same leaf size, etc.), so if you make the whole crown behave in the same way (not necessarily at the same level, but repeat the droopiness elsewhere in the crown to show that it's a recurring thing), I think it'll look more natural. And there is no sign of leaves in the background, which makes it look flat. The lack of shadow cast on the trunk adds to the flat look. If the crown extends a bit away from the trunk, the shadow should take up a much larger area of the trunk.
If you want to go with something more realistic, leaves are spread out more with gaps in between. You can see individual leaves in and around the tree crown, and you can also see branches in between the leaf clusters. This is of course more painstaking to pixel, so a simplified approach might be more suited for your first tree, but it's something to keep in mind.
Regardless of the style you're going for, I think it's important to know the shape of a real tree before you simplify it, since the shape is what makes us recognize it as a tree (it could even be a neatly trimmed tree, which is just a sphere on a stick, but I suppose it's still a kind of shape we can accept as a tree since it can exist in reality). I think getting the shape right should be your first priority before working on the shading.
