1. Content. You'll have to pay your actresses and actors.
2. A website.
2a. Web servers, hosting, maintenance.
3. Drugs. Your actors/actresses will be on them.
4. Organized crime. Whenever there is low overhead and a high profit margin, look for these guys.
5. Are you going to sell your content to other sites or distribute it yourself via DVD. If distributing, see #4.
Happy Porn-Making!