Spain is a signatory of the Schengen Agreement, so visitors from any of the Schengen member states enjoy free movement and there is no border control. If this is not your case, just apply for a tourist visa at the Spanish Embassy in your country of residence.

See information about COVID restrictions in the Plan your trip page or in the section CAN YOU TRAVEL TO MADRID FROM OTHER PARTS OF SPAIN? at the bottom of the article

Passport

In addition to the Schengen Agreement, which applies to residents of Germany, Austria, Belgium, Denmark, Slovakia, Slovenia, Estonia, Finland, France, Greece, Holland, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxemburg, Malta, Norway, Poland, Portugal, Czech Republic, Sweden and Switzerland, there are also other agreements with certain countries which simplify entry into Spain, for which it is necessary to obtain a visa, whereby the presentation of a valid passport is enough.  *Note: The last enlargement of the Schengen Area was on 19 December 2011, when Liechtenstein joined.

You can find the requirements to travel to Spain on the following Ministry of Foreign Affairs and Cooperation link.

Visa

In general, the requirements to obtain a visa include proof of sufficient financial means  for your trip, proof of return flights and a reservation for accommodation, but the procedure may vary from one nation to another, so  full details can be obtained from the corresponding Spanish Embassy, which you can find on the website of the Spanish Ministry of Foreign Affairs.

If the purpose of your trip is merely for tourism, you should apply for a short-stay visa (Schengen). This visa enables you to travel or stay in Spain and in the rest of the Schengen states for a period of no more than three months (90 days) in a period of six months (180 days) from the first entry  date in the Schengen Area.

If your visit to Spain is for other purposes, you should apply for a long-stay national visa. This visa allows you to live, live and work, study or carry out research work in Spain. All foreigners who wish to enter Spain to live, live and work or study  need to have  this type of visa, unless they are citizens of the European Union, Iceland, Liechtenstein, Norway or Switzerland.