If you head to a gym, challenge the trainer and beat the trainer, you'll get a certain badge, to become the best Pokémon trainer, gather all the badges. The Gyms are divided into the Kanto Gyms and the Jotho Gyms. If you complete all the Kanto Gyms, you will be able to fight withing the league of Kanto.
A list of all the Kanto Gyms;
A list of all the Jotho Gyms;