Generally speaking, you may study at school for a short term (maximum of six months) with a short-stay visa. Requirements for obtaining a short-stay visa vary depending on your nationality. Please contact the Japanese embassy of your country of citizenship for details.
Ministry of Foreign Affairs of Japan
Student Visa General Info
If you wish to learn Japanese for a longer period of time, (3 months to 2 years) you need to obtain a College Student Visa.
Upon the receipt of your application, WAVE USA, Inc. will walk you through the visa application process.
Timetables

Documents Necessary for the Visa Application
□ An application form (KAI form)
□ A photograph that has been taken within the last 6 months. (No hat, plain background, semi-formal clothes in JPEG format. The minimum size is 960 pixels x 1280 pixels. This photo will be used on your resident card as well.)
□ Personal records and statements of the purpose of your stay in Japan. (KAI form)
□ A certificate of graduation from the last school that you have attended.
□ A certificate of your Japanese proficiency (if applicable)
□ A photocopy of your passport (passport data pages, pages with the dates of entry/departure to/from Japan)
□ Proof that you have sufficient financial resources: Please see below for detail.
Proof of Sufficient Funds – How Much Do I Need to Apply for the College/Student Visa Japan?
One year: Fee ¥ 800,000 + Living expenses (¥100,000/month) ¥1,200,000 ≒ ¥2,000,000
Two years: Fee ¥1,600,000 + Living expenses (¥100,000/month) ¥2,400,000 ≒ ¥4,000,000
Working Holiday Opportunities
If you are from the countries listed below, you may apply for a working holiday visa. With a working holiday visa, you can study Japanese during your stay in Japan. Please contact the Japanese Embassy/Consulate of your country.
★ Australia
★ New Zealand
★ Canada
★ Korea
★ France
★ Germany
★ England
★ Ireland
★ Denmark
★ Taiwan
★ Hong Kong (from January 2010)