In case you have not yet booked flight tickets: Try to get a flight arriving at Haneda airport, not Narita. Not only is Haneda right next to the city, if you have a window seat and the weather is good you might catch a nice aerial view of Tokyo during the approach. You can then also take the monorail from the airport into the city (depending on your luggage handling capabilities it might not be a good idea, though
).
If you plan to purchase or print maps, pay attention that the map comes with the Japanese names (mostly in Kanji), which will help tremendously when communicating with taxi drivers (taxi is rather expensive), bus ticket vendors or "asking"
locals for directions.
Apropos directions: if you need to ask and communication with locals is too cumbersome, you might want to look for the nearest Koban ("police box"). Most of them have large and detailed maps of their district on a wall (although in Japanese only, as far as i have seen). Having your own map (with both Japanese and Romaji spellings) will make it easy for the staff to tell you the way, or rather draw it onto your map...
Also important: Don't forget to practice your chopstick skills before your holiday. Really, sitting there eating your meal with a fork/spoon while everybody else uses chopsticks feels akward...
Otherwise, there is not much to add to what the others said. There is so much to experience...
Oh, and pet rabbits of course:
(picture made some years ago during the Sapporo Snow Festival)