Then the quickest way would be the Red Line to Gallery Place. Change to the Yellow Line. The Yellow Line will take you straight to DCA.
An alternative--if they're doing track work, for instance--is to take the Red Line to Metro Center. Then transfer to the Blue Line going in the direction of Franconia-Springfield.
Here's a link to the rail map: http://www.wmata.com/rail/maps/map.cfm?
According to Metro, it should take 27 minutes. I'd allow at least 40 minutes. The fare is $2.70 off-peak or $3.45 peak.
Of course, you could also take a cab. That'll cost about $22.
Hope that helps.