The two standard outlet types in Germany accept plugs of types E and F or of type C. American plugs will not fit into any of these outlets without a converter, so you'll need to buy two adapters to assure compatibility with all buildings. Either way, you'll need a three-prong American outlet at one end (type B). One adapter will have a type C plug at the other end, while the other will have a type E or F plug at the other end.
Adapters simply conform your item's plug to fit the outlet. This does not assure that the voltage is correct for your item. Plugging your appliance into the wrong voltage can destroy it, so you may also need a transformer, which transforms the socket's voltage to suit your appliance.
Voltage in Germany is 220-240, and in the U.S. and Canada, it's 110-120. However, items that people typically travel with are dual-voltage, meaning that they do not require a transformer. For instance, a toaster is unlikely to be compatible, but a hair dryer or laptop probably is. Your cell phone, electric shaver, digital camera and PDA are probably also fine.
Check for an imprinting, sticker, or label on the plug. (If the device has a charger, check this instead of the device itself.) If it says "100-240v" then the device does not require a transformer. If you can't tell, look on the manufacturer's website to verify, or just pack and use a transformer to be safe. They're somewhat heavy but don't require much suitcase space.
Travel stores usually have a full range of adapters and transformers, and the clerks are likely to be knowledgeable about them. You can also purchase them online at large retailers such as Amazon, online travel stores, or Adaptec (see link in Resources). Brick-and-mortar retailers are a better bet if you're in a hurry; Wal-Mart and Target are likely to stock the transformer as well as a universal adapter, which has a variety of plug options.
If you've already left for your trip, check at the airport, as many have travel stores. If you have already arrived at your hotel and realized you do not have the correct adapter/transformer, ask the hotel desk if they have one they can loan you. (Most hotels will not be able to provide these, so don't count on this.)