You don't. They debut in Pokemon Black and White.
beat n then you have to battle him
If you mean battle him, you do that after you beat N in N's Castle.
he is at the pokemon league after you beat all four elites and N
From what I've heard, he's hiding in N's Castle.
If you mean after you find the other sages . well you cant find him ever again you will have to wait for the sequel of pokemon black and white so really you cant find N or Ghetsis ever again in your currant pokemon game
In Pokemon Black/White when you get all 8 gym badges go to the Pokemon league and defeat all of the elite four's Pokemon when you go to the champion N is there and he will take you to is castle go to the top floor and catch the legendary Pokemon Zekrom/Reshiram and defeat all of his Pokemon then Ghetsis will come to battle you.
you fight ghetsis
Ghetsis is at N's castle after you beat N After that, he is nowhere!
im pretty sure you cant find him after you beat him at n's castle
Yes, Ghetsis and N are related, but not by blood. Ghetsis is N's adoptive father. Ghetsis manipulated N by forcing him to interact with only Pokemon that have been abused by their former trainers, thus resulting in N's resentment towards humans.
Okay, soA) You need to beat the Elite 4 for the first time. The Pokemon's levels don't exceed 50B) You will ( after you go to the Champion's Room) Find N's castle. Get the items there (Rare Candy!!!) and heal you pokemon. You will soon see Ghetsis waitingC) After you follow Ghetsis, you need to catch Zekrom/Reshiram and defeat N. You battle Ghetsis. His Pokemon's levels are up to 54
In the dreamyard after you beat N and Ghetsis.