If you plan to stay longer than a tourist visa allows you start by looking at the visas Japan offers and see if you qualify.

https://www.mofa.go.jp/j_info/visit/visa/long/index.html

What visa you might qualify for will depend on multiple factors, including your citizenship, which you didn’t mention.

Americans I know who have lived in Japan went as teachers, job arranged in advance. One married a Japanese citizen.