whatever you do, don't eat out or buy prepackaged/frozen foods.
Make lots of casseroles, stews, and chili and extend them all with white rice (the cheap stuff, not uncle ben's), potatoes, or (dried) beans. For the meat, get cheap chuck roasts and chicken "leg quarters". Sometimes you can get pork roasts super cheap as well. For vegetables in your dishes, big bags of carrots, onions, and potatoes will go a LONG way. For seasoning, buy the large can of dry chicken stock and use that to season instead of salt. The large can of granulated garlic is also a good buy and can go into almost any dish.
get some cheap tupperware and for lunch eat leftovers.
mainly, you just need to learn to cook.