Hi Dotdottung,
Firstly, congratulations!
Secondly, you did not state what discipline you are in, but it sounds like you have the right idea. Make sure you know the field in which you have an interest and, of course, what YOU have written and want to do. Be clear WHY you want to do a PhD. And it sounds like common sense, but treat it like any other [job] interview, i.e. dress smart, be punctual, polite to everyone, etc.
Thirdly, do not forget that any interview is a two-way process. Hopefully, you'll get a tour, in which case take any opportunity you can to speak to other PhD students in the school and most especially, those supervised by your proposed supervisor. I have just finished my PhD (subject to minor corrections), but one thing I read on these sorts of sites four-odd years ago was make sure you get as much information on quality of supervisor as you can. THAT IS THE MOST IMPORTANT THING! I got it wrong and have spent much of the last few years suffering as a result.
Hope this helps. Good luck!