From my experience, aside from the permission from the distributors to show the anime in a public venue (like rb suggested), you're going to need:
- An appropriate venue that can hold at least 100 people, WITH air conditioning (if you're planning to throw something bigger than that, then adjust to suit)
- Proper Equipment (Projectors, Screens, Speakers and somebody on standby if technical problems arise)
- Enough chairs/tables to seat your crowd
- Somebody to serve refreshments
It also helps if you get sponsors in on the idea, and, in exchange for giving you a little something to reduce overall cost, they can sell/promote at your event. If you throw in a side event, like a Yu-Gi-Oh tournament, then a lot more people will have incentive to come.
Anyway, good luck with that.