The answer to your question is explained in detail here: http://ace.mu.nu/archives/046034.php
more specific details about what info is needed to qualify for the reward money is available at http://texansfortruth.com
(below is an excerpt from site, but there are also links on the site explaining how to submit evidence and what qualifies)
"$50,000 Reward for New Evidence On Bush's Military Record!
Texans for Truth is offering a reward of $50,000 for original information proving whether George W. Bush performed duties in the Air National Guard between May 1972 and May 1973 at Dannelly Air National Guard Base in Alabama on the terms and conditions set forth here. "