http://serebii.net/games/evs.shtml
I recommend taking a look at that.
A brief overview.
1) A pokemon can only obtain 510 EV points.
2) You obtain these points by battling other pokemon in the game.
3) Each pokemon you defeat gives you a certain amount EV points.
Note: to find what the EV values earned are, look in here:
http://serebii.net/pokedex-bw/001.shtml in the "Effort Values Earned" section.
4) A pokemon can obtain no more than 255 EVs in any one particular stat.
So, in practice. Look up what pokemon gives certain EV points you want to train in. Go and defeat those pokemon wifh the pokemon you are EV training, and keep track of the points manually to know how many you have put into the pokemon. There are various other ways to speed up this process. But those details are in that link I posted at the top of my post.