Well, in each Pokemon centre there is a girl who allows you to go to the union room. here you can communicate with other trainers and battle/trade if youd like.
To Trade, talk to the other trainer using whoever you want. Select Trade and Simply choose "COMFIRM" when you are happy to trade the selected pokemon.
You can put up 3 pokemon to choose from for trade in Black and White, and you can trade pokemon from your PC too.
Trading is quite easy, but take note you cant trade pokemon from further generations (When new pokemon come out), for example Diamond to Pearl wont work (the trainers wont appear in eachoters union rooms).
Also, trading over the Internet is possible, but you need WI-Fi and have to be lucky to find somone playing pokemon... :( , but you can organize trades with oter members here: http://pokemondb.net/pokebase/meta/515/what-is-your-friend-code . Post you friend code and organise in comments or in chat - http://pokemondb.net/pokebase/shoutbox )
Hope this helps you trade and good luck!!