Assuming you mean Photoshop CS4, then that will give you the best results but also the steepest learning curve. Elements will, as its name implies give you some of the full version of Photoshop's abilities, and depending on the version has some adjustment wizards that full Photoshop lacks.
Lightroom is intended as an image management program but can also apply changes to whole images as opposed to parts of images, Photoshop can do what Lightroom can and much more.
I'd forget completely about GIMP, it is an open-source program that sets out to rival the power of Photoshop, but suffers from an even steeper learning curve than Photoshop and has so many esoteric functions that individuals have added because they want them that it becomes hard to see the wood for the trees.
So if you have time to practise then get a book,
this is good, and start playing about with Photoshop.