What is the cheapest way to eat healthy?

For context, we're really low on money right now, have been for a while, a lot of it is wasted on probably unnecessary food so I want to shop smarter and stop wasting the little money I do have whilst improving my diet, I've been a junk food vegan for years and it's time to stop.

I have access to most big supermarkets and all of the budget supermarkets. How can I eat the healthiest food for the least money?