aperture, zoom, and distance from subject will create that "depth of field" when your subject is in focus, and the background is out of focus.
being closer to your subject can make the depth of field more shallow. stepping back and zooming in on your subject can do the same thing. also shooting at a wide open aperture will make your depth of field smaller.
as far as a critique goes, i can only view the second image. in the second image, there is too much head room (the distance from the top of the subjects head, to the top of the image) shoot more of the subjects body, and less of the negative space above her head.
edit: google about aperture, it's an interesting thing that will effect your depth of field, but also how bright or dark your photo turns out.