Like @Lightlyseared said, use all but the roots and wilted tips of the green. If you don’t want to use the white part in your fresh dish, then save them in the refrigerator or freezer to add to foods that will be cooked.
@YARNLADY Chives is a different plant, but also in the onion family.
You can use everything, and here’s a neat little trick. Cut off the root part and re-plant. You won’t get much ‘onion’ back, but you will get the long green shoots again, if you like your chives.
From my experience, as long as they’re fresh, whack them straight into soil and water regularly as you would any plant. Even just a nub of a root system works.