This is only a partial answer, perhaps CG cookie has specific tutorials.
These trees are actually quite simple, as for the modelling. Start with a cylinder for the trunk and shape it accordingly.
The leaves are actually planes with a texture. So go to another layer, and create 3 or 4 planes.
This is where you have the choice between painting in blender and using another program.
I recommand using Krita.
You’ll have to paint a bunch of leaves on your planes, as a texture, don’t forget the alpha channel!
Then, duplicate your planes and place them on the tree.
You could paint the trunk in blender, it’ll be easier. There’s no secret, it’ll take some time to have a good result if you’re not used to it.
Starting from there, you’ll have to setup the shaders.
You can achieve something like the picture you posted in GLSL shading, and maybe export it to blend4web, it’s a really nice program!
You can also have something similar in cycles with the right settings.
Well, this isn’t very detailed, but it’s better than nothing I guess. Difficult to explain everything.
My advice would be to start with basic tutorials, I’m sure all this process will appear quite clear to you once you’re more familiar with blender.