There are two routes to Kloster Andechs by public transport from München, both are hourly services. Note, not all S Bahn trains connect with the bus.
Either the S8 to Herrsching which connects with Bus 951, else one takes the mainline train (RB) from München Hauptbahnhof to Tutzing then the connecting Bus 958. Both stop at the foot of the hill leading up to Kloster Andechs. A taxi from Herrsching can save the last steep climb.
Both journeys take around one hour, however, the train is far more comfortable, especially on the return journey. The train has – facilities – the S Bahn does not.
Both the DB app and Google Maps will keep you right.