After you go through Victory Road, you reach the Pokémon League, home of the Unova region's Elite Four and Champion.
Heal Up and Prepare
Go into the domed building to the left. It's the Pokémon Center. Here you can heal up your pokémon, access the PC Box, and buy supplies.
To the right of the Pokémon Center is the Pokémon League itself. If you talk to the guy at the gate, he will step aside and allow you to pass. But when you go through, you can't come out again until you have either defeated the Elite Four and Champion or you have been defeated.
While you are inside of the Pokémon League, you will not have access to a Pokémon Center or a PC Box. You have to use the same six pokémon against all of the Elite Four and Champion. After you defeat each member of the Elite Four, you will have a chance to use healing items to heal up your pokémon.
Now is the time to buy plenty of healing items. Get Moomoo Milks in Driftveil City; Hyper Potions in the Pokémon Center; Lemonades, Fresh Waters, and Soda Pops from vending machines; stat boosting items like X Defend and X Attack; and status healing items such as Awakenings, Burn Heals, and Revives.
The pokémon that you will be up against are level 56, 57, 58, and 59. You should level up your pokémon to at least level 55 before you take on the Elite Four and Champion.
If you need to level up your pokémon more, go to the area where Doctor Logan is in Victory Road and run around near the grass until the grass shakes. Go to the shaking grass, and usually it will be an Audino, which gives a lot of EXP. For even faster leveling, give your pokémon the Lucky Egg.
If you are ready, talk to the guy standing in front of the entrance to the Pokémon League and go in to challenge the Elite Four and Champion!