Here's how you get Dialga:
1. Complete the main story: You need to finish the main story of the game before Dialga appears in the Space-Time Distortions.
2. Wait for the Space-Time Distortion to appear: These events happen randomly and can be tracked on the map. You'll see a swirling vortex in the air marking the location.
3. Enter the distortion: Once you enter the distortion, you'll be greeted by a high-level Dialga.
4. Prepare for battle: Dialga is a powerful legendary Pokémon, so make sure you have a strong team and plenty of Poké Balls ready.
5. Catch it!: Use your best tactics and items to weaken Dialga and try to capture it.
Important Note: You'll need to have caught Arceus before you can catch Dialga in Pokémon Legends: Arceus.
If you're referring to other Pokémon games, please specify which one. Dialga appears in other games as well, but the specific method to catch it varies.