Ula'ula Beach (Japanese: ウラウラビーチ ʻUlaʻula Beach), known as Secluded Shore (Japanese: ウラウラ裏海岸 ʻUlaʻula Secluded Shore) in Pokémon Sun and Moon, is a location on Ula'ula Island in Alola, located south of Route 12.
Assuming you beat Sophocles's trial, you need to travel down Route 12 and go to the near end of the route. There will be an area you can go south, and it will lead you to Secluded Shore aka Ula'ula Beach. Here is a map for reference:
You will need to go past the area that leads to Blush Area. After you pass that area, go south and you will reach Secluded Beach.