Trains are labeled by their ending station. Some lines share the same track for parts of their journey. The touristy stops are Smithsonian for the museums (yes there are several), McPherson Square to get to the White House, and Capitol South to see the Capitol building.
On escalators stand to the right so others can walk on the left. Let people off the trains before you get on. Don't hover at the bottom of escalators or in the way of train doors.
I'm about to provide this advice to a friend who has about 5 hours to kill in the city today. Anything else I should share?