S-Bahn to Munich Airport
Quick answer: S1 and S8 both connect Munich Airport to the city center. S8 takes about 41 minutes to Hauptbahnhof, S1 about 45 — since they alternate, there's a departure roughly every 10 minutes combined, and both stop directly at Marienplatz with no transfer.
Which line to take
It doesn't matter much which one arrives first — S1 runs via the west side of the city, S8 via the east, but both call at the main central stations including Marienplatz and Ostbahnhof.
Tickets
You need a ticket covering fare zone M–5 through to the city zone. The Airport-City-Day-Ticket, sold at airport ticket machines and most S-Bahn stations, covers unlimited MVV travel for the day.