What the heck is a visa? Why do I need one? How do I get one?
A visa is an endorsement placed within a passport that grants the holder officer permission to enter, leave or stay within a country for a specified period of time. Many countries (ie European Union) grant American citizens automatic visas when they enter for a specific amount of time. However, if you are staying in the country for an extended period of time, you will likely need to apply for a visa.
To check visa requirements, you can go to the embassy website for the country you want to intern in. It will be important to know the timeline of your program. For example, foreigners can travel with a valid passport in the Schengen States for up to 90 days in an 180 day period, so if your internship was less than 90 days, you may not need to get a visa. However, if you are being paid, you may need to apply for a work visa.
The embassy should also have instructions on how to apply for the visa. If you have already secured an internship, you can check with your company and see if they have any information about what kind of visa you need to get. They may even do the process for you.