Bringing your own car
Importing a car to Germany can be quite complicated and costly. If you import your car from another country, there is a number of things you have to observe. For one thing, you have to pay customs duty when importing to Germany. You also have to get your car tested and registered (which implies a variety of fees), for which you need all the necessary and genuine documentation from the country of origin. If you import from the USA, for instance, be aware you might even have to change car parts because they are illegal in Germany.
The best thing to do in order to avoid unpleasant surprises is to get in contact with an automobile club. This way an expert can give you advice and tell you what to do before and after importing your car to Germany. The biggest club is the ADAC (Allgemeiner Deutscher Automobil-Club, for General German Automobile Club).
Renting a car
Renting a car in Germany isn’t exactly cheap, but most car rentals offer a good service, such as quality cars and no-fault insurance. There is a great variety of providers and a wide range of cars. Offices of international companies such as Europcar, Sixt, Hertz, Avis, etc., can be found in most German cities. Smaller, locally-based car rentals can be cheaper though. Luckily, there are a lot of online portals for price comparison (e.g. preisvergleich.de, in German), probably the best thing to do when you plan to rent a car in Germany.