Buying reserve train tickets in Japan

Where is the most convenient place to buy train tickets. I will be arriving at Haneda airport and staying in Asakusa. I want to buy reserved train tickets from Asakusa, Nagoya, Takayama travelling 3 days later. I will not have a JR Pass and no other pass suits.

Depends on the company that operates the train you want to book tickets for. Each company has major stations which will have offices/counters where you can make bookings if you provide them with all the details (train name, departure station/date/time, arrival station/date/time). Use Hyperdia (hyperdia.com/en) to get all this information, write it down and take it to the relevant station and ticket counter. In Asakusa, I believe the nearest major JR station is Ueno (about 1/2 hour walk away), while Asakusa station itself is operated by Tobu. Keisei have a station at Ueno also.