Open up Photoshop and become familiar with the tools.

Select the paint brush tool and size (there are numerous types of brushes, so try a bunch)

Choose different colors and paint!

When ready, paint the trunk of a tree.

Add leaves (there is a paintbrush that has leaves, so do it in several stages: select different colors/shades and different sizes)

Add grass (again, in stages)

Add sky (not just blue!)