First you need all the other gym badges (1-4) Then in Pastoria City, there is a Galactic member. Talk to him and follow him wherever he goes. Eventually you come to Lake Valor. After all the talking and stuff you can go to Hearthome and battle Fantina, the ghost gym leader.
To Everyone: I am now Ragnarock. I am currently awaiting my mod transfers. I will not anwser to tree afro unless I am using this account. DO NOT ADD RAGNAROCK TO YOUR PAL PADS IF YOU ALREADY HAVE TREE AFRO ADDED!!!
[b][i][color=navy]Pokemon Diamond____Name- Luke____Friend Code-0945-8747-6687
Battle Record- _51 WIN