Everything about this can improve: Colours, forms, lighting and textures.
Composition is symmetrical which is legitimate if that's what you really want but I'd suggest to offset the tree horizontally to make it more interesting.
I suggest you find a reference photo of a tree that is clear and has a strong light source and sketch a stylised version of that.
It also helps to have another 1 or 2 good references to study on the side to give u more information about the subject and/or other ideas you might want to merge into your design.
Post the references here as well as your attempts so we can help.
You should learn alot from this simple practice.
Start by sketching the trunk and branch structure and save a copy to post at this point before you add leaf clumps.
If u click on my profile and press posts and go through my history you should see an animated gif of a process I used to draw a tree. If you save and load images like this in your software you should be able to study the basic steps I used.
Experimenting with various work flows really helps you progress instead of feeling like you're placing pixels randomly and hoping it turns out ok.
