In my opinion as an experienced traveler, NOBODY should go to a country for the first time without reading everything they can about it.

So I suggest going to your nearest large bookstore and buying a guidebook for Japan. If you don't know where you're going to be, I suggest the Insight Guide to Japan, because it has beautiful photographs and lots of information on Japanese culture. I find that reading the Insight Guide to a particular place always makes me more eager to go there.

Then you can figure out for yourself what you want to see.