A very simple, though hardly practical rule to follow perfectly is: “Avoid things with ingredient lists.”

Michael Pollan as mentioned elsewhere “eat food, mostly plants, not too much.”

“Avoid the centre of the grocery store” assuming a North American layout (which has regional variations) - the point being that the prepared foods are in the middle, the outside edges are butcher, dairy, and produce.

Some asshole once said, “Eat meat and vegetables, nuts and seeds, some fruit, little starch and no sugar. Keep intake to levels that will support exercise but not body fat.” This is also good advice, despite the source.