The visa you will require depends on your reason for being in Japan. While visitors from many countries don't need a visa for a 90-day (or less) sight-seeing based stay, the rules change if you are planning to work or engage in any kind of activity that involves remuneration.
Japan's official site for immigration should be your first point of reference for any immigration procedures related to working visas.
Generally speaking, a visa sponsor is required if you want to work in Japan. The type of work you can do is restricted to that which requires high level skills or professional knowledge. The types of activities that can be engaged in covers a wide range—from a diplomat to an entertainer—however, in each case, a visa is granted based on whether a contribution to the Japanese economy can be made or not. Private research into the kinds of business activities that are authorized and the necessary requirements to obtain a visa should be made.
If you are interested in a longer stay in Japan and want to engage in employment to supplement your travel, research the possibility of obtaining a Working Holiday Visa.
A number of countries hold a bilateral agreement with Japan to allow travelers up to the age of 30 (in most cases) to live and work in Japan for a set period of time. Many conditions apply—including nationality, the kind of work that can be undertaken, and having the funds to support a long period of stay in Japan—and these conditions continue to change and evolve. Thorough research is required to find out the latest and necessary information regarding eligibility. Start by reading through Japan's official Ministry of Foreign Affairs site about obtaining a Working Holiday Visa for Japan .
With the introduction of the Longer Stays Program, it is now possible to stay in Japan for up to one year. To be eligible, foreign nationals and accompanying spouses must be from countries/regions to which Japan implements visa exemption measures—children are not allowed to accompany. Foreign nationals must be over the age of 18, and have savings that equate to 30 million Japanese yen. If the applicant's spouse does not apply as an accompanying traveler and wishes to stay separately under this program, the applicant and his/her spouse must own savings equivalent to more than 60 million Japanese yen in total.
Subscription to a private medical travel insurance policy which covers death, injury and illness during his/her stay. The period of stay is six months. However, it can be extended to one year with a Permission for Extension of Period of Stay (available only once). It is necessary to obtain a specified visa of Designated Activities in advance from Japanese embassies or consulates that exercise jurisdiction over the applicant's domicile to enter Japan under this program.