[saaxu]

To become a permanent resident you have to live in Japan for 10 years, less if your work benefits Japan as a country, you entered Japan as a college student and got a job in Japan after graduation, or you marry a Japanese. To become a citizen you have to demonstrate fluency, literacy, employment, financial stability, assimilation into Japanese way of life, etc. Then it's up to you and your immigration lawyer to sort things out.

But you shouldn't be thinking about citizenship unless you lived there for a number of years and feel that the benefits of citizenship (primarily voting) is worth it. Permanent residency is just fine since you don't need to renew your visa, it makes it easier to get loans and credit cards, you don't have to renounce your citizenship to any other country, etc.