The Spanish Ministry of Foreign Affairs and Cooperation establishes some requirements for entry into Spain for a stay not exceeding ninety days in any six month period, according to:

• Art. 5 of the Schengen Borders Code
• Art. 25 of the Aliens Act
• Arts. 4 ff. of the Aliens Regulation

These requirements can be summarized as:

• Possess a valid travel document allowing him or her to cross borders.
• Possess a Visa when required.
• Show documents justifying the purpose and conditions of the planned stay and have sufficient means to support themselves, both during the period of their planned stay and for their return to their country of origin or passage to a third State where their admittance is guaranteed, or they must be in conditions such that they can legally obtain such means.
• Not be banned from entering into Spain or any other country in the Schengen Space.
• Not represent a threat to Spain's public order, national security, or international relations, or to those of any other member country of the Schengen Space.

Further information regarding these requirements and as well as the Visa issues can be found through this link and/or directly through the Spanish Embassies or Consulates whose addresses can be found through this link.