Basically: have a healthy diet in your house! At mealtimes and during cooking, talk about the food, how good it is, and if you want to go into details, about how it has vitamins, good fat, fibre or whatever makes it the "right" food. Also, talk about how tasty it is, hand out samples for them, have them involved in the preparation of food whenever possible.
If they cannot have a sweet, tell them why. "Well, you really had enough chocolate today. if you are hungry, how about a [insert favourite fruit/veggie/whatever here] or [some alternative]?"
Do not stress out when they overindulge on unhealthy food occasionally. Even adults do, after all, and for someone who eats generally well, that's not a problem. It's their birthday? Let them stuff themselves with cake, and tomorrow it just happens to be a veggie-day. Those are EXCEPTIONS, and days when you shouldn't bother them with food-facts.
Above all, they should just be familiar with the healthy things. They don't even need to know if eggplants are healthy, they just need to be familiar with them. Show them the food, give it to them raw (where the food allows), cooked, roasted, have them touch it... anything, really. If they have known bell peppers all their lives, they will happily eat bell peppers.
Generally: just have them eat healthily, talk about good/bad foods when the occasion arises, and be relaxed.