I used to struggle with this a lot through college. One of the best ways to save money is buying your own groceries and cooking your own meals, but I always lazy. You can get simple dinners from a Walmart or a Giant Eagle which have Alfredo that all you need is boiling water and milk, throw it on a stove stop, mix it for 7 minutes and dinner is ready. Sometimes these are as cheap as .99 Cents and they are decently filling. Add some cayenne pepper and adds some flavor to it. Time is money, some Walmart's have people do your shopping for you, all you do is go online, put it in your cart, tell them when you plan to pick it up, and boom all you have to do is drive to the walmart and pick up your groceries. This will help you save time and those random ahh I should get this, ohh! that looks good lets get that.
Also, I find Cereal was always a good choice for dinner. Simple, Easy, Quick and Cheap.