Instructions
Visit any Pokemon Center in the Johto region. If you are trading with a friend via Wi-Fi, you must register his Friend Code in the basement of the Pokemon Center, if you have not done so already. No Friend Code is required for a direct system-to-system trade.
Walk upstairs and talk to the woman behind the center counter to set up the trade. Instruct your friend to do the same thing on his copy of the game.
Follow the onscreen instructions to connect to your friend's Nintendo DS and complete the trade. You must trade one of your own Pokemon to receive a Pikachu.