Most German cities have excellent public transit. Between buses, light rail and subways, you can get pretty much anywhere you need to by public transportation. Biking is also popular in Germany, and there are bike roads set aside for cyclists. Gasoline is more expensive than in the U.S., so unless you are traveling in remote areas, consider traveling without a car.
One thing that will throws some Americans off about Germans is that the latter do not smile for no reason. If a German does not beam brightly when he meets you, it doesn't mean he dislikes you. In Germany, waiters are paid enough to live on and tipping is not mandatory. You can tip if you receive good service and it will be appreciated, but it is not required. Always carry cash in Germany, since there are many restaurants that don't accept credit. Finally, avoid jaywalking. In America, everyone jaywalks, but in Germans don't, so you are much more likely to get fined.
Although it's always nice to know a bit of the language of the country you visit, in Germany it is not mandatory. Most Germans learn English in school and, although not all of them speak it well, you should be able to make yourself understood. Germans value formalities such as shaking hands, so be sure to whenever you are introduced to someone. If you speak German, always use the Sie (formal) form unless you are invited to use the Du (informal) form. World War II and the Nazi regime are still touchy subjects in Germany. It is best to avoid the topics entirely. You can, however, talk politics, philosophy and other serious topics with Germans you meet. German people tend to prefer these sorts of conversation to small talk.