i'm not from germany, so i can't help you on where you have to buy the tickets. but i can tell you that you better buy the "stehplatz" tickets. the front rows of the big arena concerts are always the "stehplätze", while the places where you can sit are further away on tribunes (too far away to see something (only the videoscreens maybe), less of the concert atmosphere, no possibility to dance around and have fun like you will have on the other places, and last but not least they are usually more expensive).