German is the primary language of Berlin and Germany. Information can often be found in English, and sometimes in French. The 2006 World Cup led to improved information and language training among for public transportation staff. Most people under 40 speak English and there are many, many expats in Berlin, but it is important to learn the language to easily navigate the city and get to know the locals.
There are some words and accents in Berlin that differ from regular German, especially in the former East Berlin. Known as Berlinerisch, some of the most common words you hear are "Icke" for "Ich", "yoot" for "gut" and "wat" for "was". There are also some words that are unique to the Berlin area.
Schrippe = Roll
Stulle = Sandwich
Broiler = grilled chicken
There a variety of schools and ways to learn.
Language schools are plentiful and come in different styles and price points. Most offer sample lessons or let you sit-in for free before signing up. Shop around before choosing.
Private lessons are another option. More expensive than language courses, these can be very effective. Expect to pay between 10-20 euro per hour depending on the tutor's qualifications and experience.
You can search for opportunities in a local newspapers, on message boards at pubs, copy centers, supermarkets, universities, and cafes, or on online boards such as EasyExpat's classifieds' or Job Listings for tutors and classes. You can also find friends on the forum to work on language exchange.