If you are living and working in Japan, this will be easier. If not, you could start by applying for certification as an official translator. How and where you do this will depend on what country you are living in, but here is one example of how to go about it. If this doesn't apply to you, if you type 'translator certification' plus your country, in a search engine, you should be able to find the right information.

You can also get an idea of what's available by putting 'japanese translator jobs' (perhaps with the relevant country or city as well) in a search engine. I found offers like these, which may interest you. You could also try to gain some experience by applying to translation agencies - any city will have some of those, listed online or in the phone book. Finally, you could apply to any major Japanese-owned company and ask them about translation work. Hope this helps.