You've got some really great branch structure there, but pretty much everything else needs work.
First of all, the colors. The leaves will need multiple colors, and they generally need to be "scattered" quite a bit - look at the trees in Nifflas' KS tilesets.
The tree trunks: they're a bit of a funny shape, right now F is the best-looking trunk. Try to make them a bit smoother. There need to be more contrast between the two colors used for shading them; right now it's barely noticeable. You're trying to do a bit too much with the shading; keep lines thick, smooth, and simple.
The leaves' shape is obviously going to change depending on the tree, but right now A, B, and C are the only ones with a good leaf pattern. Again, they need variation in color, and could also stand to be quite a bit more dense. Use three or four shades of the same hue (probably with the same saturation as well), and spread them around according to the shading. If that makes the tree look "flat," the colors need more scattering; if the shading isn't noticeable, they need less scattering.
The step-by-step method for drawing trees pretty much goes: draw trunk shape, draw leaf pattern, shade trunk, shade leaves. Usually it's the last part that takes the most time and practice, and you'll almost certainly get it wrong the first five times or so. Drawing trees takes a lot of patience.