After the sixth badge, you have to travel south from Fortree city. This route will have many trainers to battle, and is pretty long. Once you finish it, you will be in Lilycove city. There are a lot of Team Aqua memers there. Talk to all of them and you'll get an idea where to go. Hope I helped! :)
Back in the 3rd town where your dad is hope this helps :)
look for steven on the east. talk to him. battle kecleon. he will give you a "Devon scope". go to the sixth gym. use Devon scope on the invisible barrier. it is a kecleon but it will flee. then you are accessible to the sixth gym.good luck getting the sixth badge. (tip: bring electric Pokemon along.)
you do the other gyms and the elite 4
Pokemon platinum GBA version is a hacked version and you cannot get the sixth and eighth badge.
You need to clarify which badge.
at the gym
Complete the national Dex with Jhoto region Pokemon wait a few Seconds,Minutes,Hours,days, or weeks then select the three jhoto starter Pokemon
Get the 8th badge then go to the okemon league
You can get the 5th badge from your dad in Petalburg City.
Go to the Pokemon league.
you battle your father
the ground gym
At lavaridge town
Yes, someone can help get a sixth Pokemon in Pokemon emerald. Swellow is a good Pokemon for flying types. Even though they are hard to train, dragon Pokemon make an excellent sixth Pokemon as they are "extra" but very powerful.
go get the gym leader's badge.
Fortree City ( Feather Badge ) Leader's pokemon: Altaria, Swellow, Skarmory, Pelipper
The sixth gym badge (the Marsh badge) is located at Saffron City's gym. It is in Gym Leader Sabrina's possession, so you have to defeat her to obtain it.
you beat the gym leader