Quick answer: Both S-Bahn lines from the airport — S1 and S8 — stop directly at Marienplatz in the city center, no transfer needed. Journey time is roughly 41–48 minutes.
No transfer required
Marienplatz sits on the shared S-Bahn trunk line through central Munich, so whichever of the two airport lines you catch first (S1 or S8) will take you straight there — you don't need to change trains.
Tickets
You'll need a ticket covering fare zone M–5 (the airport zone) through to the city center. The Airport-City-Day-Ticket covers unlimited MVV travel (S-Bahn, U-Bahn, bus, tram) for the day and is sold at ticket machines at the airport and most S-Bahn stations — look for 'MVV Münchner Verkehrs- und Tarifverbund' then 'Airport-City-Day-Ticket' on the machine.