Along your way, you'll get attacked by Gary, and he will test you to see if you are ready for the Pokemon League. He will be pretty hard, considering that his pokemon are at high levels. Anyway, get past the inspection building, and you'll end up on Victory Road. This cave is quite hard to solve, but you will need to use Strength a lot to push down boulders. To help you on your battles on the Pokemon League, you can catch Legendary birds.
Legendary birds are very powerful and are at high levels.
Zapdos, the thunder bird, can be found at the end of the power plant. Moltres, the fire bird, can be found at Victory Road. And Articuno, the ice bird, can be found on Seafoam Island. Catching them is quite difficult, so you'll probably need to lower than HP a lot and then put them to sleep before catching them. You need to use an Ultra Ball to catch them, and putting them to sleep is an absolute requirement.