You can travel to Potsdam by train. You may prefer to travel to Berlin first, because it is close to Potsdam and provides many international bus, train, and flight connections.
In addition, you may like to use the app "Jelbi", because it comprises Berlin's entire public transport and sharing services in just one app.
Potsdam by Train
Berlin by Bus
There is an international bus stop called ZOB in Berlin-Charlottenburg. You can then go by train to Potsdam, which takes about 40 minutes (start: Berlin Messe Nord/ICC).
Berlin by Train
International and national trains typically stop at Berlin Hauptbahnhof. You can then take a direct train to Potsdam, which takes about 25 minutes (start: Berlin Hauptbahnhof).
Berlin by Plane
From now on, Berlin has only one airport, namely Berlin Brandenburg Airport (BER). You can then take a (direct) train to Potsdam, which takes about 60 minutes (start: Flughafen BER - Terminal 1-2).