If you are in Trento, bus service is certainly the most convenient solution to reach Lake Garda.
First you have to reach Rovereto, both by bus and by train. From Rovereto you can take the bus line 332 in the direction of Riva del Garda, the northernmost point of the lake. The bus passes through Mori and Torbole, then arrives in Riva and continues towards Arco.