Da Nang's new bus station is located on Dien Bien Phu after it turns into Highway 1A. The simplest way to get there from the river is to take Quang Trang west, which turns into Tran Cao Van, veers left and meets up with Dien Bien Phu.
Take a right and the station is 2.5kms down on the left. This is where the local spur of the railway line intersects with Highway 1A just where it takes a sharp elbow turn. Traffic here is madness, so if you go out to the station by motorbike, use caution.
The bus station is one of the busiest in Vietnam outside Hanoi and Saigon. Both the north and the south of the country are serviced from the same terminal, and there are numerous departures for all domestic routes from very early morning until late at night. The exception would be international departures to Laos, but otherwise you don't really need to book ahead -- just show up and buy a ticket for the next bus. First and second class fares are available for many routes, and will buy you a more comfortable, less crowded ride.