Hey there,
There's a lot of planning involved if you are traveling for 1-2 months. Plenty to see, do, eat, drink and more.
If going for over a month, I suggest at least 3 weeks in Tokyo and its surrounding cities, another week in Kyoto/Osaka/Nara/Kobe area, and another week in Hiroshima and perhaps further south.
January is of course winter time, so going south (perhaps even Okinawa) could be nice, especially if you are into outdoor adventures which would be difficult in the cold.
I have a lot of things on my blog that could be useful (
http://smartjapantravel.wordpress.com), and I am soon publishing a book that shows the right way to experience Japan and how to do it economically.