best advices:
1. launch gimp and play around, there's nothing better than this ÷)
2. browse the help for layers and channels, and play around, try it
3. read through the startup tips, really helpful
4. visit some sites (
http://tigert.gimp.org/gimp/tutorials/, blabla)
Once you got the idea of layers, try writing some text onto a solid color blackground, and try all the 'efffects' and 'script-fus' on the text to see what happens