I am an Indian. I have a tourist visa for visiting USA and Japan. Can I travel to Japan first, tour there for a month and then travel to USA directly from Japan?. Or Should I come back to India after visiting Japan and then travel back from India to Japan?
You don't need to come back to India, because the visa does not dictate where you should start your journey.
You can fly to Japan, spend some time there, and from Japan fly to the US. For immigration, it would be just the same as if you flew from India (or anywhere else).
The visa guarantees you entry to the border point, and it does not dictate anything more (or less).