1) Go to
www.gimp.org2) Click on Download
3) Choose whichever download file is right for your computer
Mine shows links for Mac OS X because that's what I use, but I think if you use a PC it will show the download link for your computer, not a Mac.
And then you wait for it to finish downloading!
Gimp is a 100% FREE program, and 100% VIRUS FREE. It will NOT ever give your computer viruses.
Make sure you have your parents permission before you download Gimp.
If you have a Mac, it may run a little slowly and tend to crash (not often unless you have a ton of pictures open) because X11 is not the Macs native system. It works fine on PC's.