I believe there's a bus that runs direct from Tegel to Potsdamer Platz, I'm not exactly sure, because when I arrived I had to stop at the Intercontinetal first but I think that bus continues down Tiergartenstraße. In any case, there's definitely a bus directly from Potsdamer to the intercontiental, so you wouldn't need to transfer to the Sbahn. In any case, google maps is very good about telling you exactly which bus/train to take and where to transfer, I had no problems going all over the city for about a week.