My recommendation for the everyday user is to leave most settings on Auto and leave your resolution as high as possible.
If you're feeling adventurous, you can adjust your ISO if you're in low light situations. The drawback to ISO is that if you forget to adjust back to 100 or Auto, your photos will be "noisy" when they don't need to be. Noise is that pixelation you see in low-light situations.
Another user recommended Vignette from the app store. I've downloaded it but just haven't had much time to play around but it offers a ton of options and settings to add that extra something to your photos.
My best advice. Trial and error... Play around and see what works and what doesn't.