Irst thing's first, before you can get connected and start trading online, you'll have to head to the nearest Pokecenter and speak to the nice lady atop the stairs on the far right side. She'll present you with several options, but the one we're after is the second: "Global Trade."
Once you've selected Global Trade, you'll be asked to choose between "GTS" and "GTS Negotiations." Since this guide covers "GTS Negotations," I'd say the choice is pretty obvious. Choose the second option and then confirm that you'd like to "Start Negotiations."
You'll be prompted to save the game and you should do so, otherwise you won't be allowed to continue.
GTS Negotiations Trading
Global Trade Station LogoYou'll enter the elevator and be lowered into the trading chamber. The Nintendo WFC screen will appear and ask if you'd like to connect. The correct answer here is "Yes."
The next screen will ask how you'd like to choose your trade partner. If you've swapped friend codes with someone and have arranged to meet at a certain time, you can choose "Trade Rendezvous." If you're looking to trade with a random player from anywhere in the world, you'll want to choose "Trade With Anyone." Choosing the first option takes you to a list of trainers you know. Choosing the second takes you to the "Trade Wanted" screen.
On the "Trade Wanted" screen, you'll be asked to specify the level and type of the pokemon you are looking to trade, as well as the type you are searching for. These terms are vague, so most trainers just leave them at "Any Kind."
Press the "Search" button to continue. The system will search for a trade partner. This can take a few minutes, so be patient.