Dialga is a Steel/Dragon Type Legendary Pokemon and is very rare and powerful. You need the Champion Badge to find it (obtained by beating the Elite 4) and once you have the badge it can be found in Victory Road (rocky road before Indigo Plateau) but is hard to encounter, though you can up your chances of encountering Dialga by buying the "Other Legendaries" gamepass and the Legendary booster in the menu tab.