Everyone needs a visa, and you need to book a tour and be accompanied by a tour guide at all times.
North Korea is such a special country that only a few tourists get to visit each year.
The visa costs $70, plus you must book a hotel, the cheapest is about $120 per night, and finally you have to deposit $300 per person to a bank in Spain. Oh and you have to fly in which costs $300 per person (from Beijing).
Tours start at 2500 euros per person.
There are cheap tours being offered by Young Pioneer Tours starting from US$500 but they only give you 3 days and 2 nights. Most of your time is going to be spent on a train or in a hotel.
There are various travel agents on the internet that cater for North Korea, take a good look around and find one that can suit your requirements and budget.
Israeli, South Korean and US citizens have to visit with special tours and make special arrangements.