Washington DC's Regional transit system, Metrorail, does not directly serve Dulles, so you must purchase a $10 one-way or $18 round-trip ticket from the Washington Flyer Coach ticket counter, located at Arrivals Door #4 in the Main Terminal. From there, buses depart approximately every 30 minutes towards West Falls Church Metro station. To get to Downtown Washington DC, take a train from West Falls Church Metro bound for "New Carrollton". Metro fare is $3.75, with the option of an electronic ticket (SmarTrip) for $1 discount on all rides. Total travel time is about 60 minutes.
Washington Flyer Shared Van rideshare services operated by SuperShuttle and Supreme Airport Shuttle costs $29 to reach Downtown Washington, DC in about an hour's time, depending on passenger count and traffic conditions. Rideshare van counters are located on the lower level of the main terminal, down the ramps to Doors 2 and 6.