Does anyone know of a cheap connection from Hanover Central Station to Berlin Ostbahnhof?

Hello, I wanted to go from Hanover main station to Berlin Ostbahnhof tomorrow with a friend (under 18) and I don't know where to find a day ticket for a return trip that isn't too expensive. And would there be a train that leaves from there to Hanover at 11 p.m.? Please reply urgently. Thanks…