First you'll need to register on the game's website. Then you'll need to finish in the top 10 of the ultimate Ark Hunter contest that starts today and runs through July 30th. The 10 semifinalists will have their backstories fleshed out by Defiance writers, and five finalists will then be chosen by fans via Facebook. The Defiance team will then select the winner from the final five.
Neither Trion nor Syfy has specified the context in which the winning character will appear, but it's safe to say that this is one reward that you won't be sharing with millions of your fellow players. You can read the official rules on the contest page, and the requisite major arkfall events are happening now!
[Source: Trion press release]