Where do I go and what do I need to do about my visa?

Margaret, you should apply for a L type via appointed travel agencies in Japan. The required documents are your passport, filled application form, a recent passport photo, round-trip air tickets (or invitation letter). By the way, you may ask your military leader if there is any special requirement you need to get any approval for leaving Japan.

You can go to the consulate-general in Fukuoka to apply for a L one to go to China or consult more information.
Generally speaking, your passport, your application form with recent taken photo and an invitation letter from China are necassary for applyig. Besides, you need to provide other documents that the consulate-general ask. Their contact number is 092-713-1121 for your reference.