How do you surf on Pokemon HeartGold?

You can't actually "surf" in the traditional sense in Pokémon HeartGold. However, you can use the Surf HM to travel across bodies of water, which is a similar concept. Here's how you do it:

1. Obtain the Surf HM: You'll get this HM from Professor Elm at his lab in New Bark Town, after defeating the Gym Leader in Violet City.

2. Teach Surf to a Pokémon: Go to the Move Tutor in any Pokémon Center and teach the Surf HM to a Pokémon that can learn it. Most water-type Pokémon can learn Surf, but some other types can too.

3. Use Surf: When you're near a body of water, press the A button and select Surf. Your Pokémon will hop onto the water and you can navigate across it.

Important Notes:

* You can only use Surf on water, not land.

* You cannot use Surf in certain areas, such as rivers or streams that are too shallow.

* Using Surf is the primary way to access certain locations and complete some of the game's quests.

* Once you have Surf, you can use it to travel between towns and cities much faster than walking.

Have fun exploring the world of Johto with your Pokémon!