Its actually quite simple:
so your at the Pokemon centre, enter the ruins defeat the trainers and go out. Once your out follow the grassy area until you find the doctor outside the cave. heal your Pokemon if needed and enter the cave. Once inside, follow the road until you find yourself having to choose up or left, go up. After this use strength on the boulders that you can and go left after all this you enter a final passage. there your rival will battle you. after you defeat him he will give you thunderbolt. you walk out and find yourself standing to the left of the entrance to the Pokemon league....Enter and good luck with the elite four. Hope I helped.