You will need a passport to enter and exit Thailand. Your passport must be valid for the entire duration of your program. If you have not already obtained your passport, you should apply for one immediately. Information about applying for a passport can be found on the US Department of State's website.
You must mail your actual passport (not a photocopy) with your application to obtain a student visa. Travel outside the US will not be possible while your visa is being processed. It is recommended that you make a photocopy of your passport before submitting your visa application.
Currently, a visa is not required for US citizens visiting Thailand for 30 days or less.
If you are not a US citizen, you may have additional requirements you must complete before entering Thailand. If you have any questions, please contact Mike Trost at firstname.lastname@example.org.