To capture Rotom you must first defeat Cynthia the champion of Sinnoh, after doing this return to Eterna City and head into Eterna forest, make sure you have a Pokemon with HM01 Cut. Cut the bushes at the northern entrance and head towards the building you see, inside this building go to the upper level and head through the northern door. Next you will see 5 doors find the door with a Television on the inside. During night by interacting with the TV will cause a wild Rotom to appear.