To enter France for less than 90 days you must have a valid passport issued by the United States government. To obtain a passport you must visit your local post office or passport office with a complete form which you can download online. You must provide proof of citizenship, two passport sized photos as well as photo identification. If you are staying in France for more than 90 days you must obtain a visa.
As part of an effort to prevent child abduction, children entering France from other countries without their parents must have written permission from the parent(s) allowing the child to travel. This means anyone under the age of 16 must have written permission regardless what the means of the trip is (school, vacation, family visit, etc.).
A valid passport must be presented at the borders or airport if you are traveling to France from any other part of the European Union (including the United Kingdom). Unlike other countries, European Union members can stay in France as long as they desire without a visa.