Quite honestly, the biggest train station in northern Berlin is rather boring. In fact, there's not really even much of a station here, just lots of tracks and platforms and a big flat area where the station probably should be. Nevertheless, you can board long distance and regional trains here to just about anywhere, as well as the U-8 and the Ring-Bahn and the S1.
If the station itself isn't so great, the area around it is worth exploring. My favorite Berlin park, Humboldthain (along with its ghosts), is just across the way. The Berlin Wall ran very close to here, and the Bösebrücke is just up the tracks. A flood of people streamed across that bridge, which marked the division between East and West, when the wall came down. Leading right out of the station is Badstrasse, a wild street full of colors and smells and Turkish grocers singing their produce-selling songs. There's a great Lebanese restaurant a couple blocks down from the station, along with some great Turkish bakeries.