Save often. Nothing is more frustrating than having your software crash and loosing all your hours of work. Save often, use incremental names if you are worried about loosing something.

As mentioned understand layers.

Look up the tutorials for Gimp. Off the top of my head look up Torstan, RobA and Arsheesh as they all have great tutorials. They aren't the only ones but I'm not steering you wrong.

One of the best things in my opinion is to learn how to use layer masks. See jfrazierjr's tutorial on masks as understanding how to do this will save you pain in the long run. It took me a while to see the value but it's so worth it.

Have fun and don't give up. I had never done anything digitally when I started using Gimp and it was maddening not being able to craft what I knew I could do on paper but I just kept at it. It may take a while but eventually you will be able to make the software sing under your hands.