there are any number of tutorials out there - google brings up lotsa hits -
here's the first google entry I got on GIMP sig tutorials.
The best way to learn is to follow the tutorials at first until you have a good grasp of what the tools do and the effects they can help make. Then you can start to play around with the colours, tool values (opacity etc) and try different combinations of effects - little tip here: write down what you do, there's nothing more frustrating than creating a fantastic image and never being able to repeat it
Save often so as not to lose work and save versions - do something you like save it, then try altering it with different colours, effects, contrast values etc. and save as so you have your original and now some variations.
Photoshop is a fantastic tool but GIMP or Paint.NET will do for starters - if you start to feel they are holding you back then think about forking out the big bucks for Photoshop.