Potsdam is 25 minutes from Berlin by S-Bahn (line S7 from Berlin Hauptbahnhof or Ostbahnhof). The journey is covered by the Berlin ABC zone ticket, so no separate fare is needed if you already have a valid Berlin day ticket covering zone C. Regional trains (RE1) are faster but require a different ticket. Potsdam Hauptbahnhof is on the edge of the old town, walking distance from the Brandenburg Gate and the Dutch Quarter.
By car from Berlin, take the A115 motorway westward; the drive takes around 30–40 minutes depending on traffic. Parking near Sanssouci Park is available but congested in summer. Most visitors find the train far simpler.