Before you can travel abroad, you must obtain a passport. U.S. passports are issued by the U.S. State Department. The passport application, while straight-forward, can require significant processing time depending on your situation and State Department backlog. It's important that you begin this process as soon as possible if you're considering studying abroad.
If you already have a passport, many airlines require your passport to be effective at least six months into the future.
More information regarding the exact steps, forms, documents, and fees can be found on the State Department passport website.
If you have a passport from a country other than the U.S., consult your consulate for more information.