Yes, the blue or yellow line will take you there. The airport stop (officially called Ronald Reagan Washington National Airport) is right after Crystal City if you're coming from downtown Washington.

I believe there are walkways to the main terminal at both the north and south ends of the Metro station. If you're going to Terminal A (Northwest, Spirit, Airtran, Midwest), it's probably a 5-10 minute walk, or you can take a shuttle bus that comes every few minutes.

Check out the DCA terminal map to figure out where you need to go: http://www.metwashairports.com/reaga...terminal_map_2