http://europa.eu/travel/doc/index_en.htm
For non-EU citizens
Passport
You will need a valid passport.
Visa
There are 39 countries whose nationals do not need a visa to visit the EU for three months or less. These include Australia, Canada, Croatia, Japan, New Zealand and the United States. The list of countries whose nationals require visas to travel to the United Kingdom or Ireland differs slightly from other EU countries. Apply for a visa from the consulate or embassy of the country you are visiting.
If your visa is from a country fully applying the Schengen rules, it automatically allows you to travel to the other Schengen countries as well. Moreover, if you have a valid residence permit from one of those Schengen countries, it is equivalent to a visa. You may need a national visa to visit non-Schengen countries.
Border officials in EU countries may ask for other supporting documents such as an invitation letter, proof of lodging, return or round-trip ticket. For the precise requirements contact the local consular services of the EU country in question.