Hello there!
So, leaves. I personally like to break down all of my work into basic shapes. I often see leaves as being triangular. The main issue with figuring out how to shape them, is deciding what you actually want this to look like. I recently have found out that there is a way to "switch" pixel art form cartoony to more detailed styles. It all originates from the shapes that you think of while you are drawing. So, to make this simpler, triangles will look more detailed and defined while circular leaf patterns will tend to produce more of a cartoon style.
Now on to actually learning leaf textures:
Step 1: Defining those shapes! 
What I did here was actually draw out some leaves at different angles. The trick to this is to provide enough angles to have a nice variety of leaves, and to keep them readable as leaves even in a small size.
Step 2: Adding the leaves.....
In this step I thought of the different brightness of the leaves as different layers. I did
not copy and paste the leaves from step 1. Instead, I kept those shapes a reference and redrew them where it was appropriate. When you are creating these layers, it's best to think of them as a mountain. The base (darkest layer) has to be much wider than the others, in order to support the rest of them.
Step 3: Stackin' dem layrzzzz
If these layers are the correct sizes, you should be able to place them on top of each other and see the final result. One thing that I found very important about the leaf placement was thinking of them as a checkerboard. Having the leaves only placed in diagonals with each other helps define the triangular form of the leaf, along with keeping illusions of large leaves from occurring.
I do not have a mini-tutorial on the other shape options of leaves, but I hope this helps!
PS. Dear god dithering, please no. This is wayyy too small of a tree and it would be inappropriate
