What is the best way to start eating healthy things for someone who only eats junk food?

In the beginning - make gradual changes. Your digestive system wouldn't cope well if you just suddenly cut out everything you used to eat - in favor of totally healthy options.

A different contributor:

WRONG. Only the reverse is true. Your body will thank you immediately. It's like stopping smoking. If you're capable of doing it suddenly, so much the better.