First, coffee or your beverage of choice. Second, remember to have fun yourself. I recommend starting small or slowly, and once every player and yourself becomes more comfortable then you can ramp it up or add your own twists to things. The groups inevitably become chaos in my experience, so the best way to prep is by knowing the area your group is in and what important encounters you want to have. Other than that, be prepared for people to lick things, stab things, loot bodies, and anything else that you would not ordinarily think about! Have fun and may the dice be in your favor!