Im working at a salon right now, and although I'm not a stylist I know a little about em.
if your hair is longish already you dont need extensions. Dreads work best for africian americans, but caucasians can do it also. The best bet would be to go somewhere to get them done, but you can do em yourself if you want. Just buy some dreadwax and section off pieces of hair, braid and then rat them with the wax, the wax contianer might have directions on it also.
There's also stuff you can buy for more temporary 'dreads' too. It gives the similar effect, but washes out at the end of the day.
hope that helps. you'll have to show us some pics if you end up doing them.