Almost everything in a shop is packaged for two to four people, and almost every recipe is written for four. Cooking for one is not a smaller version of cooking for a family; it is a different problem, and most of it is a shopping problem rather than a cooking one.
Buy loose, buy small, buy frozen
Loose vegetables let you buy one onion rather than a bag of six. Frozen peas, spinach, berries and fish are sold by the portion by definition — you take what you need and the rest stays frozen. For anything you can only buy large — a loaf, a block of cheese, a bunch of herbs — plan where the rest goes on the day you buy it.
Cook dishes that improve, not dishes that wilt
Stews, curries, soups, ragùs and baked pasta are better on the second day and freeze well. A stir-fry or a salad is not. Cook a large batch of the first kind deliberately, eat it twice and freeze the rest in single portions. Cook the second kind only in the amount you will eat now.
Learn three meals that use half an ingredient
Half an onion, half a pepper, half a tin of beans, one egg. A fried rice, an omelette and a bowl of pasta with whatever is left will absorb any of these. Having three reliable formats means the second half of everything has somewhere to go.
Use the freezer as your second household member
The reason a family wastes less is that food gets eaten before it turns. A freezer does the same job: it holds the half you cannot eat today until you want it. Freeze bread by the slice, cheese grated, herbs chopped in oil, and tomato purée in spoonfuls. Every one of those is a thing that normally gets thrown out in a one-person kitchen.
Shop in ones, cook the things that keep, and freeze the half you will not finish. Cooking for one wastes food only when it is treated as a scaled-down version of cooking for four.