Reshiram is a Fire/Dragon type Legendary Pokemon and is found like all legendaries as a 1/1000 chance. You can find this Pokemon by going to the Mausoleum of Origins (obtained in Taxi after beating guy in tent at Fuchsia City) and you can find it inside the Mausoleum anywhere in the grass, though you need to have Kyurem and a fire type Pokemon in your party in order to encounter him.