A good photo starts by getting it right in the camera. If the shot is out of focus, nothing you do in GIMP will save it. And of course, if the subject and composition are nothing special, then the photo will never be something to write home about. Do the best you can, but have realistic expectations.
If the colors are dull, try increasing contrast. Preferably mid-tone local contrast. There are many ways to do this. Start here:
viewtopic.php?f=23&t=3573You can install the Clarity plug-in, which is an easy way to punch things up.
http://registry.gimp.org/node/32316When you're in the mood to read something complicated but brilliantly useful, try this:
http://blog.patdavid.net/2014/08/clarit ... tones.htmlA subtle vignette can help a flat photo look a little more exciting, by drawing the viewer's eye in toward the center.
It takes a long time to learn the ins and outs of GIMP. Be patient, try lots of tutorials and keep coming back here for advice.