The Hauptbahnhof (Main Station) in Schwerin is just 100m or so from the Pfaffenteich (lake) and just north of the main city centre. The station is within easy walking distance of the main sights however, with the train and technical museum (Mecklenburgisches Eisenbahn- Und Technikmuseum) actually at the station itself, and then the cathedral and old town being to the south of the Pfaffenteich. On a nice day, you may like to take one of the boat tours across the lake for a gentle way to get to know the sights.
Services from the station are frequent and comprehensive; Rostock and Hamburg can both be reached direct in under an hour and the UNESCO World Heritage Site of Weimar in approximately 35 minutes.