To reach Garda Trentino region by bus there are direct connections from Trento and Rovereto (Autobus Trentino Trasporti). You can calculate your route with the Trentino Trasporti buses clicking on this link.
From Verona, Desenzano, Brescia and Peschiera (A.T.V buses) and during the summer (from mid-June to end of September) also from Venice (A.T.V buses) there are both direct buses and with some possible changes. Here you can have the timetable of every bus that gets to Riva del Garda, Arco and Torbole.
You can also choose the FLIXBUS service, the new carrier that has lots of low-cost transportation services to Trento and Rovereto. A cheap and green alternative route. Here times to and from Rovereto.
The tickets can be purchased online or at the tourist information centers in the Garda Trentino area.