either drink tap water (works well in Germany) or cheap mineral water.
Buy noodles, rice, lentils and other basic foods on sale.
Plan and do your weekly shopping, because less shopping means less unnecessary spending.
Buy vegetables and fruit at the wholesale market or on sale.
Meat is very expensive, if you really need it, you have to calculate everything again.
Prefer food that can be stored well.
Last but not least I recommend my cheapest lunch:
Noodles with tomato sauce (3 cans of chunky tomatoes, onions, Italian herbs, salt)