http://www.wowhead.com/?talent#jfVMzZZfMhIxcdx0ux0uIqo is the standard Unholy build to start with. As with all DK builds, start by applying diseases. You then perform a Scourge Strike, followed by two blood strikes to create Death Runes. Do two Death Coils to burn RP, and start your Unholy Blight debuff. By this time, your runes are up and you can perform more scourge strikes. Pestilence should always be used to refresh low diseases, as Icy Touch and Plague Strike take away a Scourge Strike. Simply repeat as runes come up, and you can do good DPS.
Edit: Omitted URL and had minor grammatical errors
Edited, Oct 5th 2009 3:33pm by Lichbourne