There is no direct bus to the Arenal area from Santa Teresa.
Shuttle companies in the Santa Teresa area usually have schedules to run that route. (this would take about 6-10 hrs depending on which route they take). Check Tropical Tours web site, Interbus, and Montezuma Expeditions for shuttle prices and schedules.
To drive the route north, across the Friendship bridge, to Tilaran, around Lake Arenal and into La Fortuna will take about 6 hrs. more or less depending on if you make any stops or not.
If you were to take a direct bus, you would have to take the same bus line you take from San Jose, get off the bus at the airport and catch a cab or regular bus into downtown Alajuela and then get on the direct bus to La Fortuna/Arenal from there. This would likely take about a day (10-12 hrs)
Edited: 4:02 pm, May 21, 2013