It's a 90 minute train journey, and the train runs every hour. It's likely to be packed, though - so make sure you reserve a seat.

I take it you're flying into HAM? Public transport into town isn't brilliant (no train link yet - it's under construction). There is a dedicated bus service to Hauptbahnhof (Central Station), I've used it a few times and it's fine. Probably not worth getting a taxi - it'll hardly save you any time.

Don't know how you'd get to the stadium from Berlin Hauptbahnhof, but knowing Germany there is bound to be adequate public transport.