What do you do after you beat your dad?

Answer 1

In Omega Ruby and Alpha Sapphire after you've beaten your Dad; Norman the Gym Leader your next step is to head east of Mauville City so you can get to Route 118 however prepare a Pokemon that knows Surf and once you get to some water you will need to use your Pokemon to get across it and then you can make your way through the route until you run into Steven and while you chat with him one of the Legendaries (either Latias or Latios) will appear to warn you and Steven of trouble and the two of you will fly off to Southern Island at which point you will have to fight off some grunts from either Team Magma or Team Aqua depending on your game and then the Legendary that you were protecting whether it was Latias or Latios will offer to join your team at which point you will need to accept the invite but by doing so you will also need to select a Pokemon to send back to the PC if you already have a full party.

Answer 2

Head east, across the river to the east of Mauville, after obtaining Surf from Wally's parents.