After defeating Volkner, head north from Sunyshore City onto Route 223. You'll need to use Surf to navigate this water route. There are several trainers here, mostly Water-type specialists. Battle them to gain experience. You'll eventually reach the Pokémon League entrance.
Before you can challenge the Elite Four, you must traverse Victory Road. This is the final dungeon before the League, and it's a long and challenging one. You'll need Surf, Strength, Rock Smash, and Rock Climb to navigate it. There are many strong trainers and wild Pokémon like Onix, Steelix, and Machoke. Make sure your team is well-trained and you have plenty of healing items. At the end of Victory Road, you'll reach the Pokémon League building.