Let me warn you .... modding is not the way to go if you have no previous experiance with game programming. Sure, you will be able to get some fancy effects to the screen, but mostly you would be just twiddling with settings... and that's not really making a game.
But the choice is yours so:
Content: http://gpwiki.org/index.php/Tools:Content
Programming : http://gpwiki.org/index.php/Tools:Programming
Also checkout the documentation for the SDK that came with the game.
I would recommed you start with some really simple games like pong or tetris. You will learn a lot during the process.
I did my first game with minimal knowledge using DOS graphics in about 2 weeks. It was a snake clone (I never did get around to doing a text game). It's not very hard.. and then you can move to higher level games quickly once you have an understanding of the basics.
Not to say modding is not game development, I've seen a lot of mods that really are good and do significant changes to game play. But I still hold playing with settings and art of a game is not really game development.
Have fun ^.--