Berlin, Germany II: Around Potsdam

I went on a tour to Potsdam which is the largest UNESCO site in Germany. I almost missed the tour because I took my own sweet time getting myself out of bed. I was still really tired and kind of lazy to go out. I managed to catch the tour group at the platform where a lady exclaimed, "Oh! My roommate!" I was pretty sure she wasn't in my room but she went on to tell me there was a free dinner last night and how breakfast was not too bad this morning. Potsdam is around 30 minutes away from Berlin, we got into a train then a bus. The guide brought us to quite a few famous buildings but I remembered nothing about them (sigh), the Dutch quarters which was really pretty and lastly the tour ended at Sanssouci Palace and Park where we were left to explore on our own. I remembered the Sanssouci Palace and Park better because I walked around by myself with a map. I had a bit of trouble finding my way back to Alexanderplatz but somehow a (what appeared to be abandoned) train station appeared and I hopped on it and reached Alexanderplatz safely.

How to get there? 
I've got no clue because I was following the guide blindly. The tour was EUR12 for students.