Aside from doing steroids, losing weight, or getting chin implants, there's not much you can do. Growing a beard (if you can grow one) might help a bit.
You need to find a contour shade that works for your skintone, and apply it under your jawline using a brush and blend down into your neck. if you do it right it should look like a natural shadow
Your jaw structure is determined by bone... Can't really change it (short of plastic surgery), but as others have said, growing beard can give the illusion.
