Most people prefer to start modeling and then add the textures and I think for regular objects that is usually the way to go. But when modeling a tree, it is simply hard to judge the appearance with ugly squares as placeholders for the leaves. So when creating plants I mostly create the textures first, at least some preliminary materials - fine tuning can be done as last step.
We will start with some work on the leaves...
I will use Photoshop during the following steps, but of course all this can be done with every other paint application too, like GIMP for example.
Because trees tend to have maaany leaves, we usually have to go with a simple leaf geometry - in many cases a simple square object is used and often this simple object will be used not only for a single leaf, but even for a cluster of leaves. This of course depends on the model itself, but the work to be done is more or less the same.
The following video will show some approaches on how to create a leaf texture with alpha or transparency channel. If you did some planning before taking the photos, then this should be a simple job: separating the leaf (or cluster of leaves) from the background.
If you don´t have scans or photographs of leaves at your disposal, you can get some here: Download leaf scans
Some of the tools you could make use of:
selection based on color (wizard, wand)
manual lasso selection
painting the selection