Coaches are numbered, and somewhere on the platform you will find the Wagenstandsanzeiger - a plan for each train using that platform showing you which coach will stop where. If you look up you will see big boards with letters - A, B, C, D etc - above the platform; you will see the same letters on the Wagenstandsanzeiger so just check where your coach is in relation to the letter boards and then go to the right area to await the arrival of your train. (Don't worry if you're in the wrong area, though - as you can always walk along inside the train to get to the right coach)
Grossraum means it's an open coach - as opposed to one that has been divided into separate six-seat compartments. Long-distance trains usually have both types.