I wanted to post a few different subjects. These are my first. I have no previous experience. Just picked up a D3100 two weeks ago and have been ready and studying and just went out the other day and shot these
As for the female portrait, I recommend a light fill flash. I shoot in 50/35mm and use a fill flash that gives just a 'kiss' of light and the adjustments are done in Adobe Camera Raw. In this outdoor set you can see where I got the fill flash right and some images were it was not strong enough - https://www.flickr.com/photos/lautermilch/albums/72157704440960924 .
The bridge image would be better 2-3 later during the day as the sunlight is a bit bright.
The cotton ball I would visit all the apertures for a shot that like to get the best one possible.
Ditto with the paying mantis.
Very well done for early efforts! I like the bridge the best. The light is a tad harsh, but I think with some work in Lightroom or other editing software you could bring up the shadows. I'd like to see the top of the roof rather than having it cut off, and also more of the river in the shot. Of course, I don't know what your shooting situation was, so maybe you couldn't shoot any wider.
Anyway, you have a good eye. Keep shooting, that's how you get better. You are off to a very good start!