Learning to eat healthily is a skill. It's a separate skill to learning to eat less. I don't think any amount of junk food would get you adequate nutrition. So

Learning to eat healthily is a skill. It's a separate skill to learning to eat less. I don't think any amount of junk food would get you adequate nutrition. So reducing the amount of junk food is a great first step. You can take a multivitamin, it'll be enough to survive. Later, once you've established a decent calory baseline, you can start to optimize your good for nutrition.