As a personal trainer I have dealt with this issue many a time with clients, family and friends.
The quantum shift begins, from my experience, when both the following things occur.
Firstly there needs to be a overwhelming desire to eat properly. Be it for an over weight condition, doctors advice or other...
Secondly when you begin to properly exercise, on the completion of each session there is an amazing sense of accomplishment accompanied by an endorphin rush. An almost enlightened feeling. Its this feeling that will help you make better decisions with food. You will notice your body changing and so you will want to reward it with the right food options.
Of course having junk food in the house is a problem especially when it comes to temptation. So the best option is to not buy it in the first place.
There are many foods that can satisfy a sweet, tasty craving.
I resort to hummus as an example or make a veggie juice. Other times I make gluten free rice pasta with lentils instead of mince beef....
There are so many possibilities...
Healthy eating becomes habitual so don't expect it to happen straight away.
My favorite saying at the moment when it comes to unhealthy food options is "I can eat that but I choose not to".