I would just hate to get on the wrong train and end up in Calcutta
I'm assuming you will take the S-bahn to the airport; the S-Bahn doesn't go to Calcutta.
In five departures from MUC since 2001, I've left left from Munich 3 times and taken the S-Bahn and from Freising twice and taken a bus.
Twice I stayed in the suburbs and took the S-Bahn from a suburban station; one time I stayed on the east side of the old town and took the S-Bahn from Karlsplatz.
There are two S-Bahn going through the central tunnel below Munich to the airport. The S1 goes around the west end of Munich to the airport; the S8 goes from the east end to the airport.
There are basically four S-Bahn stations near/in the old town, Hauptbahnhof (tief=deep), Karlsplatz, Marienplatz, and Isartor. Three of them are one track level stations with tracks going both directions. Marienplatz has two track levels, each with a track going only one direction.
Sound like Marienplatz will be the closest station for you. It's a three level station. The uppermost level, one flight below street level, is a mezzanine with ticket machines, etc. The S8 to the airport leaves from the next level down (Gleis 1); the S1 to the airport leaves from the lowest level (Gleis 2).
You want to go down the stairs to Gleis 1 and take the S8. 95% of the time, that will get you to the airport the soonest.
Here is a walk through of the Marienplatz station.