C&C per req:
(in future, please post your images vertically and number them to facillitate response)
1. Nicely captured; good expression, nice exposure, but the background is really, really distracting. This sort of shot needs a smooth, even, homogenous background about 1/3 - 1/2 stop darker than the model's face. I'd prefer your shooting position to have been slightly lower.
2. Slightly under-exposed, and shot from too low a position. This sort of shot should be shot at the model's eye-level.
3. I like the symmetry of the model and the tree limb, but she's significantly under-exposed.
4. Fill light was needed here; your background is MUCH brighter than the model's face, and as the eye is always attracted to brighter areas over darker, the blown background is pulling the eye away from the model's face.
Overall it's a good starter set. One major issue you need to work on is your white balance. All of these have a strong green cast, which is easily correctible in post (esp. if you've shot in raw). Watch your shooting position and try and get yourself even with the model's eye level.
Just my $00.02 worth - your mileage may vary.
~John