There's an enormous choice of language schools in Japan: you’ll find that they have different focuses and can be of varying quality. The Association for the Promotion of Japanese Language Education was set up to ensure that the standards of courses offered to foreign students is upheld, and you are recommended to attend schools which are recognised by the association.
You can choose whether you wish to follow a short-term or a long-term program. The classic short-term programme in Japan is during the summer, which is convenient since you’ll usually be able to stay in Japan on a student visa. Few schools offer short-term programmes with a specific focus, such as business Japanese or conversation, but rather the classes are divided up according to level. If your ultimate aim is to get your Japanese to a good enough level to enter a Japanese university or work in a Japanese language environment then you should be looking at a long-term study programme of a year. Long-term study will require a student visa.