Some bullet point advice:
- first make sure you really want to move to Japan. I'd suggest doing at least one trip (stay as long as you can) to really make sure you want to live and work in Japan.
- You need academic credentials if you want to get a visa in Japan (unless you come as a student). At least a Masters' Degree.
- You need to build a network/contacts locally in order to make yourself visible, and know their needs. That's probably the hardest part, but something you can start addressing while visiting the country during a trip.
- Be in touch with the latest technologies in the West, as most of the web development in Japan is years late. That's where you can bring some positive insights and practices to local businesses.
- Japan is a Windows/Mac/iOS world so you better focus on these environments to have some value on the market here. Android is still small here, and Linux is virtually unknown.
- Learning a bit of Japanese would not hurt, but it's not mandatory.
Let me know if you have questions on particular points...