I'd do what Mark Cuban did with his Cost Plus Drug Company, which sells everything on it at cost + 15%, but for other essential products as well. I'd open up all types of stores that sell necessities - petrol, groceries, medication, plants, technology items, books, etc - at cost + whatever the lowest margin mark up I could get away with. The goal would be to drive prices down everywhere, not just at my stores. I'd also spend big on lobbying/campaigning for better legislation and regulation around essential goods and services.

Low cost public housing would be another area. Buying up land to build low cost housing estates with all the facilities and amenities needed.

Obviously it goes without saying I'd set up everyone of my family and close friends for life too.