Here's how to get it:
1. Defeat the Elite Four and become the Champion. This is necessary to unlock the ability to catch Legendary Pokémon.
2. Travel to the Dark Cave, located in the northeast corner of the Johto Region. It's near the entrance of the Victory Road.
3. Enter the cave and make your way to the back, where you'll find Raikou.
4. Prepare for a tough battle! Raikou is a powerful Electric-type Pokémon with high stats.
5. Use your best Pokémon to capture it. Use a Poké Ball with a high capture rate, and consider using moves that can weaken Raikou and lower its catch rate.
Important Note: Raikou is a roaming Pokémon, so it might not always be in the Dark Cave. If you don't find it there, try checking other locations around Johto like the Ilex Forest, Ecruteak City, or the Goldenrod City Game Corner. You may need to try multiple times before encountering it.