looking for collaborative game design tools

[Spookifire]

As my luck would have it, my college roommate is a computer science major interested in video game programming, and I am a digital media major interested in game design. We've become really good friends over the past year and we would like to begin work on a project just to see if we can make a game. The problem is neither of us know of any tools that let us collaborate on one project or how the pieces fit together. He's made entire basic games out of java, and I've made a couple of levels, but we're looking for information on how to put my design ideas and levels/characters ect into his programming. Any help would be greatly appreciated. Where is a good starting point for us?