You can reach Oberammergau by train from Munich central station (München Hauptbahnhof), you have to take the train headed to Innsbruck (a regional train that departs on daily basis) and switch trains in the Murnau train station (the train to Oberammergau is the only other train in the station), the trip from Murnau to Oberammergau shouldn't be a problem, since Oberammergau is the last station to be reached.
You may also reach Oberammergau by car, you would have to go to south Munich in order to the take the Autobahn A-95 south to the Ettal exit. From there on the B-23 to Oberammergau.
Getting around Oberammergau shouldn't be a problem since it's a relativly small town. You can get around (as most locals do) walking, but you can also take the bus (there are a couple of bus stops in town), you could also get around by car, and like most of youth by bicycle.