I recall http://gimp-savvy.com/ being handy when I started with gimp a few years ago.
Basically, just start simple, don't try to do complicated stuff right off the bat. For me anyway, it was very much just a continual process of learning.
The biggest bang for the buck is playing with layers, I found. Very useful. E.g. making comics: text on one layer, speech balloons on layer below that, art layer(s) below that.
Also be aware that the transform tool's scale does a crappier more pixelated job than image scale. (Unless they've finally fixed that.)
My (several years old) short page of info:
http://russcon.org/gimpinfo.html
Maybe give Photofilter a whirl before getting stuck into a doctorate in the Gimp.
http://www.photofiltre.com/
It's totally free, loads very fast, downloads quickly and does most things you're likely to need.
(If you are editing GIFs enable RGB mode or you cannot do anything.)
Re: Iain's last bit of advice. More generally, always work with the image in RGB (or grayscale) mode, saving your intermediate versions as XCF (or whatever file format your chosen software uses), and learn about RGB vs palette (and how you use should save the final presentation image as JPG for RGB vs PNG (or GIF) for palette) if you don't know already. I see many sins and blunders committed by people who don't grok that stuff. 
Hmm. I did some reading up about RGB vs palette. I didn't realise I had to convert back to palette... I wonder why it's not automatic?
http://skaiste.elekta.lt/Books/O...ign/
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