Prepare by familiarizing yourself with the work being done by the lab-have a look at the lab's website and take a look on pubmed. Also, try and think of some relevant questions of your own that you would like to ask. Be prepared to tell them what you think you want to be doing in five or ten years' time. Everyone knows you can't plan exactly what you'll be doing then, but they will want to know that you have thought about it and have some sort of plan.
Good luck!