If it's peak season (Golden Week, Aug, or Oct) or a weekend, it's best to reserve a room in advance, which you can do by calling a lodging either directly or through a travel agency in Tokyo. Off-peak, you can make a reservation upon arrival at Nikko Tobu Station, either at the tourist information counter or at the accommodations-reservation window (tel. 0288/54-0864; Mon-Fri 9am-5pm, Sat-Sun and holidays 8:25am-5:55pm), both of which are familiar with the area accommodations and will make all arrangements for you. Note that the charge for booking a room at either of these facilities is ¥100 per person.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.