You are Going For an Internship
Internships can be a great way to experience Bangkok, but they can be a bit tricky when it comes to getting the correct visa. Working in Thailand, even voluntarily and unpaid, generally requires a work permit and a Non-Immigration B Visa, but internships may be exempt in some cases.
The deciding factor is whether or not your internship is a required part of your education. If it is, then it will count as education and not work, and a Non-Immigrant ‘ED’ visa will suffice (read the You want to study section). If on the other hand your internship is not part of your primary education, then you will need a Work Permit (You have a job and need to relocate section) and Non-Immgrant B visa.
In any case, it’s always a good idea to consult your local Thai embassy or consulate with visa matters.