There exist the following ways to reach Trongsa, which is located right in the centre of the country.
The closest airport would be Paro Airport. From Paro, one could then reach Trongsa by cab.
From Gelephu in Southern Bhutan, Dungkar Transport plies buses to Trongsa on Mondays, Wednesdays and Fridays at 7:00 (arrival time is roughly 17:30). From the border town of Phuentsholing, Sernye Transport plies buses to Trongsa on Tuesdays, Thursdays and Saturdays at 6:00. (arrival time is roughly 19:30). From Thimphu in the Western region, Sernya Transport plies buses to Trongsa daily at 7:00 (arrival time is roughly 15:00).
The attractive spots in Trongsa are situated close to each other and can hence be covered by foot. However, one could opt for hired cabs but this might prove to be slightly on the expensive side.