Regardless of the kind of tree you would like to make - this tutorial is a really good starting place. It's by Nauris.
https://c10.patreonusercontent.com/3/e30%3D/patreon-posts/1667196372481049860.png?token-time=1508976000&token-hash=LWUCxVVkWvq1LVzPydR8QPvWEVzvmZeo-hEXboCDqwQ%3DAbout your tree - You need to texture the leaves. You have the right idea with the shading and light source from the top left. The right of the trunk can have darker colors to give the illusion of cylindrical form. Add the actual ground color beneath the tree and blend the roots to the ground.
All in all - it's no easy feat. Be patient with it and don't focus on one area alone for too long. Also about your perspective, is this for a top down game or a platformer. If it's top down, you'll need to imagine as if you are floating above the tree and think more about how much of the truck you will see. In essence, the form is kinda like a mushroom, more flat on the top. But if its for a platform game disregard that comment.