Lighthouse Entrance
Quest starts at NPC Yasmin. Below map with way from pokemon center in Olivine.
Yasmin conducts research on electric type pokemons. She asks you to bring for her few items: 1 zap plate, 10 flash sphere, 15 pikachu's tail, 10 raichu's tail and 5 electric tail.
Zap plate can be obtained from electric type pokemons.
Flash sphere can be obtained from electric type pokemons.
Pikachu's tail can be obtained from pikachu.
Raichu's tail can be obtained from raichu.
Electric tail can be obtained from electabuzz.
Once you gather requested items go back to NPC Yasmin and you will be able to go into lighthouse as a reward, that is located nearby.