good luck mate, my friend runs a promotion company (we're 16 haha) and he gets bands to play for free, fronts up the money for the venue himself, then takes costs and the rest of the ticket money goes to cancer reasearch. pretty simple jobby.
your best bet it is probably getting it at a local venue, because getting it in a park or some otehr public place renders you with a whole otehr bunch of problems, like council permits and noise levels and picking up litter after and all sorts of stuff, getting it in a venue means all that stuff is covered when you place the deposit etc, you just have to worry about promoting the thing.
good luck, good cause
