It is best to alert the airline that you will need assistance, so they can decided how to help you get to the terminal. The airline will also help you get through the immigration area if you need help. In general, the airport is well marked with English signs, and most of the staff can give you guidance.
All doors are automatic opening, and anywhere there are stairs, an escalator or ramp also exists.
If you think that you will need special assistance in the airport, it is best to rely on the airline which you came in to Tokyo on. If you are in a wheelchair and going through the airport on your own, you should go through immigration at the office in the center of all of the regular lines. At this location there is a low counter. After going through immigration, you will need to go downstairs to pick up your baggage. There is an escalator or stairway to get downstairs to the baggage pick-up, and airline staff can guide you.