This will show academic and post-high school benefits to forensics. Also, be up front about what kind of program you intend to create. Your principal will be much more amicable if you clearly lay out the costs and time commitments necessary, including your planned tournaments. Finally, you shouldn't be afraid to be tactical with your presentation. Mentioning a few rival schools that have successful programs can never hurt your cause.
Every principal understands local competition very well.
The principal is the only person's permission you need to begin setting up a forensics program. Once you're on your feet, however, you may want to seek out the superintendent to squeeze money from the school corporation's pocket. There are two ways to access this information. The best way is to surf the net. If you can't find the appropriate site, you can call a local school that participates and ask for a schedule.