Many Tourist take the Subway to Theresienwiese, when they are visiting the Oktoberfest in Munich.
But so do very, very many tourists! Theresienwiese U-Bahn Station is super crowded.
Personally, I take the Subway to Goetheplatz and exit the subway there.
From Goetheplatz it is only a couple of minutes walk to Theresienwiese.
If you ask me, take the subway to Goetheplatz and walk from there to Oktoberfest.
Have a great stay in munich!