Press "ctrl+n" in Photoshop to create a new file. The settings are as follows.
Use the brush tool to draw the outline of the tree.
Create a new layer and use a thicker black brush to paint the trunk.
Change a brush and set it as follows.
Use brown to depict the light and dark parts of the tree trunk.
Use the blur tool to add light and shadow effects.
Use the smudge tool to draw out the thin branches at the end of the trunk. Pay attention to the effect in the upper right corner of the picture below.
Create a new layer and use a large green brush to paint the shape of the tree crown.