What do they like?  

I try learn my son eat all the kind of food since he was too small.After I believe is too late,if you don't give to him all the kinds of food when grown up is difficult he eat from everything.Of course he has some favorite food like fry potatoes.I decide one day at the week cooking for him whatever he want eat,and the rest days all the other kind of food.I never cooked second food because he doesn't like what I prepare.

My teenager will eat anything I put in front of him. My three year old on the other hand is very picky.
Thinking back my teen also went through a picky stage around this age. He wouldn't eat meat so I used to put tofu in soup and tell him it was cheese! LOL
At any rate..... I can't "make" my little one eat anything that he doesn't want to eat. It just evolves into a power struggle if you try to make them eat. Here is what I do to try to insure he eats something at mealtime. I don't let him have any snacks after lunchtime so he will be hungry at dinner. If he doesn't eat his dinner, I will leave it on the table and when he asks for a snack I tell him he has to eat his dinner first. Sometimes this works, sometimes it doesn't. Our pediatrician says not to worry about it though because a child will not starve themself. If he doesn't eat in the evening he makes up for it at breakfast. :)

Both my kids are good eaters. They dont eat much sweets (depends on what I give them). But my son adores carrots. He'd eat them every minute of the day if he could. While my daughter is having a few sweets in a bowl, he'll be having carrots.