Go outside with your notebook. If you can, find a bench or picnic table with a view of a tree. Get comfy, open your notebook and take a stab at describing the tree. (Hint: it's not as easy as it sounds.) Once you've described how the tree looks, try using some of your other senses. Is it a windy day? Is the tree making a sound, for instance? Is it a flowering tree? Does it have a scent?
When you've finished that consider different ways of creating moods in your writing using description. Let's say you were writing a spooky scene. How could you describe the tree in a way that would make the scene more sinister? There are endless ways to create atmosphere with something as simple as describing a tree. Can you think of three different moods you could create by describing your tree differently?