Hi!
The best way to find the shortest itinerary would probably be this page:
http://www.fahrinfo-berlin.de/fahrin....exe/en?L=bvg&
There are several ways to go from TXL to the Mariott Hotel, all of them are a mix of subway and bus and take about 30 minutes. The name of the station closest to the Marriot Hotel is "Potsdamer Platz".
Regards,
Stefan
P. S.: The name of your departure airport is "Flughafen Schönefeld", or "Schoenefeld" if you have to avoid the umlaut...