As far as I remember, there's nothing you need before hand at all, your orthodontist should supply you with everything. Just have some pretty strong painkillers ready for once they're fitted (yes, it will hurt, sorry!).
As for what you can't eat, your orthodontist will go through it with you, but broadly speaking anything likely to break the brace, or get stuck in it. For example, toffee, crunchy apples, crusty bread, chewing gum are all supposedly not allowed, but to be honest you can get by just by cutting stuff up into small pieces.
Just a note, it probably won't hurt while they're being fitted, or immediately afterwards. It will start to really throb about half an hour in, after your teeth actually start moving. You'll most likely have a lot of trouble eating for the first couple of day. I got by on jelly and mashed bananas.
Good luck with it anyway, it's not that bad.