Supplies I used:
Watercolor brush (size 14)
Cold-pressed watercolor paper
Photograph of a tree (for reference)
In order to improve your chances of painting something that looks "real," it helps to have a visual reference for what you're painting. You can go outside and paint the scenery in front of you (that's called painting "en plein air") or you can use a photograph you've taken. Or you can find a photograph on a website that offers public domain photographs, like Unspalsh.com. (Important artistic note: don't refer to someone else's photograph if you don't have permission to do so.)
When you begin painting, lightly sketch an outline of your tree on paper that's designed for watercolors. I've had good luck with Arches cold-pressed cotton paper, which can be found on Amazon or most art supply stores. I also like Winsor & Newton watercolor paint for reliable, pretty colors.
When you begin to dab paint on the paper, start with the lighter colors first.
Give the color a chance to dry before you apply the next layer. However, if you want the watercolors to mix with each other, you can apply the next layer while this one is faintly wet.
I painted the tree trunk and the ground while I waited for the yellow paint to dry. And then I added layers of darker paint blobs to the tree's leaves.
When watercolors dry, the colors will lighten significantly. I used a waterproof black pen and a brown colored pencil to add details to the tree trunk and limbs. Here's the final result:
I've linked some of the supplies I used, in case you're ready to paint some watercolors. (Some links are Amazon affiliate links, which pay a very small percentage of money to me at no cost to you.)