If you want to paint landscapes, it's worth spending some time painting studies of individual trees and various species of tree. It allows you to focus on one thing only, to become better acquainted with a tree's characteristic form, colors, and textures. It also builds up your visual memory, so when painting from your imagination you can add an oak, poplar, gum, etc, to a composition relatively easily.
Spend time observing different trees in real life, rather than only from photos, because you'll see far more. Sketch the pattern of branches and leaves, note where shadows fall within the tree itself on the leaves and branches as well as the shadow cast on the ground or nearby trees. You may find it easier to focus on the negative space between the branches (as I did in this potplant sketch).
Take an individual leaf and sketch both the front and back, which not only differ in texture but often color too. Note the overall shape of the leaf. When painting distant trees in a landscape this shape can be used as an outline for a small tree, as the overall shape of the leaf often echoes the overall shape of the species.
The first step is to select paint colors for a tree...