The best and cheapest place for sandwiches and baguettes is on the burger king side, right above platform 13. You'll spend half the money here than at the places in the wandelhalle.
Check out all the trains :) There is a food court, - I recommend the Indian or the sushi (pricey). There is a supermarket on the second floor (Edeka). Confusing to find the metro
If you need to recharge your smartphone, go to KFC, they've got two sockets (just ask the staff where they are). If you want to eat something in the dead of night, the McDonalds is open 24 hours.
btw: Moin Moin does NOT mean "Good Morning", you can say it all day. If you want to feel and act like a local (like "senf" suggested), and if you are under 40, you might want to use "Moinsen" instead.