Personally I use photoshop, but the gimp is a program that can reach photoshop level and it's free. According to what i heard, it's quite difficult to use it, but there must be plenty of tutorials out there to help you.
To make a graphic, I'd say you need a picture (->lol), know what you'll make an which size it'll have (like an icon is 100 on 100 pixels, you crop your image etc.).
For tutorials how to use the gimp, I can redirect you to:
Gimp_users,
the gimp &
Gimp_resources These three are LJ communities. just go through their posts and you'll certainly find great stuff to help you. =D
On
the site of the gimp there are tutorials as well. If you google "the gimp tutorials" you'll find some awesome sites too. Like
here or
over thereLast advice, have a stroll in deviant art. They have categories for tutorials and resources. Just type 'the gimp' in the search bar and have fun. =D!
I hope this answer will help you. I'm not familiar with the gimp, but I've seen people making amazing things with it.
The key of graphics is practice a lot and try out all the functions to see what happens. It's pure trial and error. XD!