The documentation you need to enter Spain will depend greatly on where you are coming from:

• If you are a citizen of any nation in the European Union, Switzerland, Norway, Iceland, or Liechtenstein, you will only need your National Identification Document or a current passport. For minors, the National Identification Document must be accompanied by written parental permission.
• If you are from Andorra, Argentina, Australia, Bolivia, Brazil, Brunei, Canada, Chile, Costa Rica, Croatia, El Salvador, the United States, Guatemala, Honduras, Israel, Japan, Malaysia, Mexico, Monaco, Nicaragua, New Zealand, Panama, Paraguay, the Republic of Korea, Romania, San Marino, the Vatican, Singapore, Uruguay, or Venezuela, you will need to show a valid passport to enter Spain. Your stay may not exceed 90 days.
• If you are coming from any other country, in accordance with EU regulations, you must apply for a visa at the Spanish Consulate for your country. You must also show an official passport. In this case, Spanish authorities may require proof of accommodation, reservation confirmation by an official travel agency, a return ticket, tour agenda, or personal invitation. Likewise, you may be required to provide proof of sufficient funds, to cover your stay in Spain. As the conditions stated above may vary, in case you need a visa please contact the Spanish Embassy or Consulate in your country of residence to ensure you meet all requirements before traveling.

It is the responsibility of each participant to obtain a visa if necessary.

The average time for obtaining a visa can be between two and six weeks, depending on the application date.

Check the Spanish embassy and consulates in your country:
https://www.exteriores.gob.es/en/EmbajadasConsulados/Paginas/index.aspx