It depends on whether you want to go by either:
- Short Term (Visiting as Tourist, Possible short term study)
- Long Term (Working or Other)

Short term is very straight forward, depending on your country you are issued generally 90 days visa upon arrival in Japan. If you are one of the few 5 or 6 countries you can extend this further by another 90 days (upon acceptance from Immigration at their discretion). Within the 90 days Visa you are free to travel within the country but are not permitted to work

Long term is alot more complicated and falls under many Categories such as Working Visa,General Visa,Specified Visa etc. Obtaining these visa's (which duration range anywhere from a few days/months to 1-5 years) requires you to have a purpose to fulfill the Immigration Offices decision to give you a visa, i.e Working or Study etc.

Depends entirely what you plan on doing, Short term and visiting the country isn't a problem or difficult. Getting work and living in Japan is different kettle of fish.