Set up camp near a desert (better than plains since it's less likely to rain there) and go out in the night with bow & arrows, sword and dirt. When you see an Enderman, clear out the immediate area of other monsters, dirt tower up to height 3 and look at the Enderman or loose an arrow at him. Keep shift pressed so you don't accidentally fall off.
Endermen are relatively uncommon. I usually only find 1-5 per night. Try to hunt them in wide, flat open areas at night. And if you can, try to get a Sword with the "Looting" enchantment to increase the amount of Ender Pearls you get from a single Enderman kill. It can be a long, boring process but it is rewarding.
I often fight Endermen in caves. Because they love to teleport, it makes it hard to fight them outside at night. But in caves, it is easy. Also, when fighting it, HIT IT'S FEET! It almost never teleports away if you hit it there.