You need off camera strobes or speedlights plus a way to trigger them, typically a trigger that sits in your camera hotshoe plus radio receivers on your flashes.
It's too long of a topic to reply here. Check out http://www.strobist.blogspot.com/, lots of good info here. If you're a book person I highly recommend picking up this, the single best introduction to off camera flash I have found http://www.amazon.com/This-Is-Strobist-Info-Photography/dp/032180354X