If the flight you are on is the Olympic Holidays charter from Manchester, then you certainly can't book seats or meals directly with Germania, you'd need to do it with Olympic if indeed it can be done at all. They should be able to tell you what will be possible in 2014.
You don't get meals as such with Germania. They do a similar thing to Ryanair or Easyjet, ie. toasted sandwiches which you order on board. The ham and cheese toasties are ok but avoid the "breakfast" offering as friends didn't think much of it.
I usually buy a Boots meal deal at the airport, a sandwich, dessert bar and big bottle of water for not much more than they charge for the water only on a charter flight