To do this, you will need to defeat the following gym leaders:
* Roark, the Rock-type gym leader in Oreburgh City
* Gardenia, the Grass-type gym leader in Eterna City
* Maylene, the Fighting-type gym leader in Veilstone City
* Crasher Wake, the Water-type gym leader in Pastoria City
* Fantina, the Ghost-type gym leader in Hearthome City
* Byron, the Steel-type gym leader in Canalave City
* Volkner, the Electric-type gym leader in Sunnyshore City
After you defeat all of the gym leaders, you will be able to challenge the Pokemon League in Victory Road. The Pokemon League is a tournament of four battles against the Elite Four and the Champion.
The Elite Four are the following trainers:
* Aaron, the Bug-type Elite Four member
* Bertha, the Ground-type Elite Four member
* Flint, the Fire-type Elite Four member
* Lucian, the Psychic-type Elite Four member
The Champion is Cynthia, the Dragon-type Champion.
After you defeat Cynthia, you will become the Pokemon Champion of the Sinnoh region. You will be rewarded with the Champion's Certificate, and you will be able to use the Fly HM to travel anywhere in the Sinnoh region.
In addition to becoming the Pokemon Champion, you can also complete a number of other side quests in the Sinnoh region, such as catching the legendary Pokemon Dialga, Palkia, and Giratina, and defeating the villainous Team Galactic.