Obligatory posting of
the tutorial with the Secret of Mana 3 tree.
I'll try be concise here.
Broken down to the most basic shapes, trees are basically a cylinder with a sphere on top. So when you shade the leaves, you need to treat the entire form as a sphere, with heavy shadows along the bottom and highlights concentrated in one frontal area rather than along the top, following the outline.
But of course shading one giant sphere won't look like a tree, so, like the tutorial says, break it up and several smaller spheres. Here's one I did a long time ago, which I never actually finished but that makes it a lot better for getting my point across-

You can see that the entire form follows the shading of a sphere-

With each smaller sphere being modified to fit the section of the bigger sphere it's on.
As for the leaf texture, avoid outlining the leaves like the plague. With something that small, you need to imply detail rather than draw it. Starting with the base spheres, you can sort of mold the leaves out like clay, having shadows back where the leaf is closer to the center and a highlight at its tip. I actually just worked using the SoM 3 tree from that tutorial as a reference. Look closely at how it shapes each leaf and try to replicate it, then try using those techniques to create your own.