This FAQ from our friends over at GimpKnowHow will walk you through the basics of the software.
More Tutorials:
Awesome Photo Manipulation in GIMP
Crazy Insane Photo Manipulation in GIMP
Enlarging Images with Liquid Rescale
GIMP101: Possessed Eye in GIMP
How to Add Text Behind a Picture
How to Install GIMPShop Brushes
How to Make a Stylized Emo Effect in GIMP
How to Make a Colorful Line/Wavey Effect in GIMP
How to Make a Loading Bar in GIMP
How to Remove and Insert People
How to Resize a Picture in GIMPShop