Note: This is just a tutorial, I didn’t go crazy about looking too good, but is better than nothing.
First of, you arrange your Work Space.
Then you select a nice brush wich will allow you to make some nice shapes with it.
For this tutorial I picked Caligraphic one.
Now, Imagine an empty tree shape [no leaves], good.
Start drwing that shape, doesn’t have to be perfect, you can adjust it later if you want.
Create New Layer
Start adding some branches, don’t go too crazy with them, since this is just a quick made tree.
Good, after you arrange everything however you wanted, merge the layers.
Duplicate the tree shape, just in case you make some mistakes.
We will use Burn and Dodge Tools for shading.
Start first by adding Shadow, just a quick one, where you think the light cannot reach too well.
Now, Pick the Dodge Tool, and make some highlights and of course the bark texture.
Then pick up again the Burn Tool, and add the Dark areas, to give some kind of depth to the texture.
Now you want to start working on your leaves.
Select again a nice brush, for this one, I selected again the Caligraphic one.
Press F5 or Window>Brushes
Check Shape Dynamics and Scattering[important]
Play with the setting until you’re satisfied.
Leaves should be Dark to Highlight, make layers for each of them.
Start making the shape you want, then add lights and highlights on top of the dark ones.
Don’t go too crazy, 4 - 5 colors are enough.
The brach having the leaves more to the back where the light cannot reach that good, you should press Ctrl+U, to turn the Luminosity a bit down, to give a nice depth.
This is just One of the Many posibilities of making a tree, but this one is simple enough for everyone, since you just need Imagination or an image as reference.
The rest is just a play with the brushes and settings.