How do you get castform in Pokemon HeartGold?

You can't get Castform in Pokemon HeartGold! Castform was introduced in Generation III (Ruby, Sapphire, Emerald) and is not available in the Generation II remakes, HeartGold and SoulSilver.

If you're looking for a weather-changing Pokémon, you could try to catch a Snorlax in the Ilex Forest. Snorlax can learn the move Body Slam, which has a chance to trigger the "Sunny Day" weather condition.