Take S1/S8 to Hbf. Your hotel Sofitel is right there.
Too little time for castles, leave that for next trip when you have more time. Capitalize on what Munich has to offer (a lot). Grap some tourist info in the tourist office (ask concierge at Sofitel) and use the subway/bus/tram to explore the city. As for biergarten, the Augustinerkeller is near your hotel.