The Pokéwalker is an item that comes with HeartGold and SoulSilver. It's basically a pedometer; it counts how many steps you've walked as you walk throughout the day. It's shaped like a Poké Ball, has three buttons, and a monochromatic screen. You can transfer a Pokémon to it once you have gotten at least two Pokémon on HeartGold or SoulSilver, because you must have one in your party and one to deposit from your PC.
To put a Pokémon into the Pokéwalker, go to the main menu of HeartGold and SoulSilver, where you press Continue or New Game. There's an option there labeled “Connect to Pokéwalker”, press that. Next, press “Go for a Stroll”, and select a Pokémon from your PC. Put them into the space on the right, and verify that you're going to put that one into the Pokéwalker. You can then select a Route to take a Stroll on. The first time you go for a Stroll, you can only go on the Routes Refreshing Field and Noisy Forest. Finally, you'll have to connect your Pokéwalker to the DS. Just point your Pokéwalker at the DS card and press “Connect” on the Pokéwalker.
After Walking for a While
After you've walked around for a while, you will start to notice that you're collecting Watts on the Pokéwalker. You obtain 1 Watt for every 20 steps walked. After you obtain 3 or 10 Watts, you can try to get items by “dowsing” them and try to catch a Pokémon with the Poké Radar on the Pokéwalker. In the first few Routes, all you can really get is berries and weaker Pokémon, but later on, you can catch Pokémon that are rare, as well as harder to obtain items. You may catch up to 3 Pokémon, and obtain 3 items; after that you'll have to replace one that you got before. Another thing to note is that after collecting Watts on the Pokéwalker and connecting it back to your DS, you will unlock more Routes depending on how many Watts you've collected overall.
Every step in the real world with your Pokéwalker that you take will give your Pokémon in it one Exp. Point. Your Pokémon that is in the Pokéwalker will also grow one level if you have it in there long enough, and it will get happier by walking with it.
To connect your Pokémon back to HeartGold or SoulSilver, do the same thing you did to place a Pokémon in the Pokéwalker, just go to the Pokéwalker menu. Select “Return from a Stroll”, then you can connect the Pokéwalker to the DS. After you do this, you will see how many Watts and steps you've gained since putting your Pokémon in the Pokéwalker. If you walked enough, it will also level up one level then.
While walking with you Pokémon, you can connect to another trainer that has a Pokéwalker. Simply press “Connect” and hold the Pokéwalkers about 2 inches apart. Once you do this, you will both receive an item, and the other person's Pokémon team will be saved to your game for batting in the Trainer House in Viridian City. You may only visit the same trainer once per day. There's occasionally a Wi-Fi event that you can download that will let you get a new Pokéwalker Route.
The sprites on the Pokéwalker are the ones used in Pokémon Diamond and Pearl, so if you liked those a lot, you'll enjoy putting your Pokémon into the Pokéwalker.
Have fun walking with your Pokémon!