Janine: natural light at the park

1. is nice, catchlights and all. If there could be more range between the white and black her tat would show better.
(lots of fiddling with curves may do it)

2. couple of hot spots, its a nice composition but the portrait guys are going to point out that not only is it an armpit shot its also up her nose. (I think she has an attractive arm pit.. [grin])

3. Best one, playful. I would put her in the upper right third of the image and let the extra grass be more or less "dead space"

This one, would be better in color, I think

I don't like the contrast (or lack of) in any of them. Might be my laptop, might be me... [shrug]

Lighting is fine.  It's best on your third one.
The first one doesn't have much light in her eyes, but it works fine for me. 
None of the lighting is dynamic and interesting or perfect portrait lighting... but it's fine, it works.  Good job.

Lauren, I will be honest.  I shoot in P mode (basically auto but you can use exposure compensation) outside.  I use manual in the studio or in tough situations.  I believe that camera makers spend a lot of time and effort when developing the way a camera's auto functions work and I doubt I can do a better job unless I am using a light meter, which essentially is what my camera is using.  I have tricks.  I zoom in and expose for the skin, I am good at guessing how to compensate for the exposure based on the fairness or darkness of their skin. 
I told that long story to tell you that I don't really know what you should have shot at.  As Mike and the sunny 16 rule state, F16 is good for sunny days.  With it being cloudy and later in the day I would have used an auto mode, IF the shutter speed wasn't fast enough I would have shot in shutter priority to get the speed and let the camera pick the f-stop.  I still keep that ISO at 200 if possible.
When doing portrait work I like to shoot with my aperture wide open, for better depth of field (blur that background if you can).

That's how I do it... you don't have to.  Keep practicing.  Your images have potential, they aren't bad.  I have seen a lot worse when people are starting out.