Spend your time doing all those things you've been meaning to do for ages but haven't got around to... you're not going to have much time to do them over the next 6 or so years!
Joking aside, congrats on getting on to a PhD. You can read up on things like 'what I wished I knew when I started my PhD' etc, so you know what to expect. Also try and find out what training will be available when you start (might be on uni website) and reading a few odd things in your field. But really you don't need to do anything, you can do all this when you start as the first stages are very slow.