If you are a U.S. citizen traveling to a foreign country, you should have a passport, but you may or may not need to get a visa. Go to Embassy.org, which has links to most of the foreign embassies located in Washington, D.C. The specific country sites will tell you if you need a visa and how to get one.
U.S. travelers to most European countries, such as France and Norway, do not need a visa to visit for less than three months. Visas are required for certain countries, including Russia and Turkey.