You can't really thicken hair. Whether hair is thick or thin depends on how many hair follicles one has and you can't make a scalp create more follicles. And if the hair is fine, you can't make it any more coarse - that's just the way it grows out of the head. You can, however, take greater care with the hair so that it doesn't break so easily and that's best done with a detangling rinse. Also, blunt cuts will make hair look thicker.