Follow Galactic Grunt until he battles you.
if you mean platinum you just have to go to the hearthome city gym and beat the leader
The fifth gym leader in Pokemon platinum is crasher wake but you get the fourth badge.
after beating the fifth gymleader Wake, you go and use surf to go from Juplife City to Canlave City.
beat the gym leaders
Go beat the fourth
go south of veilstone city into pastoria town then go into the gym make sure your Pokemon are healed because your rival is going to battle you
Beat the gym leader
umm...you beat the 7th gym
Go to the veilstone gym and beat maylene.
beat gym leader Candice
beat the heart home gym
with fighting type pokemon