How old? My son is 9 and about 2 or so years ago I decided to let him choose what he wanted to eat or not. At the store I would offer him something and he would either put it in the basket or push it away. Everything he's chosen, he's eaten with no problem.

Try offering fruit or veggies first. My son is 17 now and has a limited list of veggies he likes. He likes most every kind of fruit. He tends to like chicken nuggets and pizza, so I try to cater to those tastes every so often.