Register your friend code with someone you want to trade with and register theirs. Then go to the centre room in black/white and select trade when they appear.
For a standard world trade with someone you don't know, go to the right hand room in black/white and enter. Put down the Pokemon you want and what you're offering in return. Then it's a waiting game.
It's the same process in heartgold, soulsilver, diamond, pearl and platinum, but you have to go to the Global trade center, instead of a regular Pokemon center.