It isn't hard to get some basic improvements in your photos, even if you aren't using a whoop-de-doo DSLR camera. In GIMP, you can do a couple of things that can help your photos look nice and you don't really have to know anything about "Levels" or "Saturation".
Case in point:
This photo is SOOC (Straight Out Of Camera).
Now in GIMP, I go to the "Colors" menu, scroll down to the "Auto" function and choose "White Balance."
Next, I go to the "Colors" menu, scroll down to the "Auto" function and choose "Color Enhance."
Sometimes the Auto functions like White Balance and Color Enhance don't have nice effects; try them and see if you like it, but you don't have to feel locked in to the results. Here they weren't so bad, so I kept them.
Finally, I go to the "Filters" menu, scroll down to "Enhance" and then scroll all the way down to "Unsharp Mask." This one's one of my very favorite features. It will pop up with a little box and you can slide the sharpen right or left. For the purposes of this little tutorial, I just used the Default amount of unsharp-masking:
The difference is subtle, but it tends to bring things into very sharp focus. It won't focus a picture that's actually out of focus to start with, so don't try to retroactively do that. But an already-focused picture can still be sharpened a little, sometimes, and this feature is a good one.
There, now, that didn't hurt, did it? Here's a side by side of the first and last images: