No boots are required, everything is done in trainers. I didn't receive an explicit kit list in the joining instructions for either Briefing or Main Board, but you can use common sense to work out what it is.
Take trainers with good grip, especially for climbing the assault course wall, one kid ended up with a busted face because his trainers slipped down the front of it, hilarious! It was also rather apparent watching other PO's slipping off things during the command exercises wearing heavily worn trainers, which is bad for your feet any way if your doing proper mileage in them.
In general just relax your briefing is just as much for you to have a look at the military as them having a look at you.