Dittany is a magical plant that is used to heal wounds. It appears in the Harry Potter books and movies, but it is not featured in the video game adaptation of the first book.
If you are looking for a healing potion in the game, you can find Essence of Dittany in the Herbology classroom during Professor Sprout's lesson. However, this is not the same as the actual plant, just an extract of it used in a potion.