As mentioned, they are all underexposed to one degree or another. Some are badly under exposed. Some not so badly.
So it looks like you need to learn more about exposure, and how to use the rear LCD, specifically the histogram feature.
Camera Exposure: Aperture, ISO & Shutter Speed
Understanding Digital Camera Histograms: Tones and Contrast
Understanding Digital Camera Histograms: Luminosity and Color
Which of your D5100's 3 metering modes did you use? (D5100 users manual page 68)
What shooting mode did you use? (D5100 users manual page 61)
Have you read about
Exposure Compensation in your camera user's manual (PDF file on the software disc)? (D5100 users manual page 70)
For minimizing reflections in car windows, and to add some saturation to blues and greens, put a quality polarizing filter on the lens.
You posted 18 photos. That's good for getting some very general comments, which is what you have gotten.
18 is way to many for specific critique or comment on each one - they are not numbered/named/easily referred to individually.
At just 2 minutes per photo, someone would spend 36 minutes to C&C them all.
Which version/release of Photoshop do you have? -
http://www.cambridgeincolour.com/photo-editing-tutorials.htm
If you want to edit, I recommend you shoot Raw files instead of JPEG's.