To secure an internship aboard, it is often necessary to find a company that sets-up the internship. Internships are commonly set-up through a university for its students. Some agencies require a fee to secure a placement.
Several papers have a helpful classifieds' section.
Also check out EasyExpat's Job Listings.
If you know anyone in Berlin get in touch! It's not necessarily what you know, but who you know. Friends and family, or any contact you have in your desired business may know of a position. There are also professional social networks like Linkedin which allows for people to make business contacts. You may also use EasyExpat's forum to meet other people living and working in Berlin and make business contacts.
If hired through an agency, the agency usually guides the intern through the process. Most interns receive a Student Visa OR Cultural Exchange Visa depending on their program. For more information on visa, consult the section on Passport and visas.
As part of the EU, job seekers can access the European job mobility portal EURES. This resource provides information about job vacancies and the labor market. It also provides information on the living and working conditions, and a CV posting service.