I just got to Japan about a week ago. I dropped everything and moved here with not that much money.. Since I'm on a tourist visa I only have 3 months unless I can find a job that will sponsor a working visa.. but luckily I already have 2 interviews lined up with companies that are willing to sponsor a visa if they hire me.
I don't have a ton of experience yet to say for sure.. but I have heard making money is easy if you speak English (by doing private English conversation lessons). It's getting a work visa that is hard.

I recommend you to avoid Tokyo at all cost. Frustrated living for foreigners.