Where to start??

I don't know, most people don't want to join a team of someone who hasn't had a successful project before, so you may just want to develop it yourself. Nobody really wants to form a team over just some written concepts and an idea, usually a full design document is needed (or an already complete demo). This is why freelance game designers don't do too well unless they have experience.

edit: However I did see a thread in Help Wanted asking for a designer/writer thingy, so I stand corrected.

i think that intially you really have to prove yourself with some works done by yourself. For example a simple RPG game using some downloaded sprites. I myself am new to this industry, and I'm planning to start working on a simple game. I'll try to do my own sprites then try to make a simple demo. Maybe, this can help you.

You'll probably want to make at least some small projects by yourself, or join some small teams, in order to get some experience and start building a portfolio. Modding existing games can sometimes be a good way to achieve this, as it can take out a lot of the work involved.