If you don't have one...
Your passport is probably the most important document you'll need; no passport, no flight. If you don't already have a passport, now is the time to apply for one - it currently takes 10 weeks to apply for and receive your passport. Applications and costs are available at U.S. post offices (or you can click to the Dept. of State's passport website for detailed information). You'll need to provide two passport-sized photos of yourself, so call around to local photography shops to get these pictures taken.
If you have one...
When does it expire? Your passport should be valid for at least six months after the date you'll return to the U.S.. If your passport expires before that date, renew it a.s.a.p. Again, U.S. post offices have forms and instructions for passport renewal.
If you need to get one, quickly...
If you need a passport a.s.a.p., check the web for companies that can expedite the process (such as www.acepassportservice.com or www.passportsandvisas.com). They charge a substantial fee, but they can get passports in as little as 24 hours.
Regardless...
Bring two copies of the signature/photo page of your passport to your program's first orientation meeting. If you're a student from another university, mail the two copies to your faculty director by the date of the first orientation meeting. If we don't get these copies, you won't get your plane ticket.