There are many ways to trade.
- You can go to a Pokemon Center and go to the trade place and trade via two people in all Generations
- You can go to the world trade center in Jubilife City and trade with people around the world in Generation IV games.
- You can't trade over Wi-Fi in Gen I-III. You can only trade in Wireless Clubs.
- Also Find in Buildings
- In Generation V, it's possible to trade via the C-Gear pretty much anywhere, not just in the Pokemon Centers. This also includes the Pokemon League against the Elite Four. The only restriction is that nothing with an HM can be traded.