Like all Nikon DSLRs your D3100 has 3 light metering modes you can choose from.
For making portraits Center-weighted or Spot metering works best,
See pages 79 - 82 of your D3100 Reference Manual. The manual is a PDF file on the software disc.
Next read about White Balance on pages 87 and 88.
You D3100 should have been set to Shade.
I edited your photo.
I corrected the white balance and globally (the entire photo) sharpened the photo using the Photoshop CC 2014 plug-in Adobe Camera Raw (ACR).
Using CC 2014 I cropped the photo so her camera right eye is on the upper right rule of thirds power point.
I then selected just the girl and the tree trunk using the Quick Selection tool and with the selection active I added an Exposure adjustment layer. I added 1.25 EV of exposure.
I then inverted the selection so everything except the girl/trunk was selected and I subtracted 6 EV of exposure.
So at that point the total lighting ratio adjustment was 7.25 EV.
All my other edits were then local edits to her facial features.
I lightened the whites of her eyes, sharpened her eyes, nose, lips and the hair around her face..
I added vibrance to her iris and lips and lastly I added a very narrow black border.