If you get motivated, try using more than 1 color in the branches sprouting from the trunk. It will make your piece more interesting.

Thank you. 

I'm really new at doing this. I'm trying to improve, but I don't have lots of motivation.

I'm going to fix that.

The two greens are so similar that they look like the same colour unless one zooms in. I recommend either replacing them with a single colour, or increasing the contrast between them.

When you're working, it helps a lot to keep a preview of your piece open at all times that has it at 100% zoom, so that you can quickly see whether the details you're working on even show up. It'll help you spot colours that are too similar and details that are hard to understand.