I saw a British fellow approach a kiosk attendant in Metro and ask if he could buy a ticket. (Answer: a very loud NO.) I keep thinking about the fellow. So here are some tips:
1. You buy a ticket from a machine, not a person.
2. It costs extra to use a paper ticket -- about $1.
3. SmarTrip® cards are the D.C. equivalent of London's Oyster Card or Travelcard. You buy it and add funds. You can buy them from machines -- in stations and at CVS stores -- and through the mail. You can register them for a deposit. When replaced, the replacement value is added, minus the registration fee.
4. Tickets and SmarTrip® cards are needed for both ENTRY and EXIT to/from stations. Don't lose yours.
5. Metro attendants open and close stations, provide station security, and may (!) offer assistance, but they are not solely there as customer service personnel. They can be grumpy and disinterested.
6. Tracks post train arrival times on signs. During rush hour, you can expect a train in 3-5 minutes (barring problems). On weekends, during the dreaded "single tracking" [from repairs], there may only be 3-4 trains an hour. Make sure your children use the restroom before entering during those times. You may be waiting for a bit. They allow bathroom access only if inclined and/or during an emergency.
7. Fares are stepped according to distance and times traveled. Rush hour (M-F) costs extra.
8. One day passes for touring are available, but please understand that they have time restrictions. Don't show up at 8 a.m. thinking you can use your one-day pass.
9. The Metro system is currently plagued by repairs ( "single tracking"), frequent downed escalators, and downed elevators, some of which are quite steep. Replacement bus service is provided when stations are closed. You may find an elevator is out, an escalator is out, and a station is closed Look at the WMATA site for information about service updates, and you can also subscribe to their Twitter feed. Stay flexible in your planning. If you have limited mobility, allow for greater travel time and consult the WMATA site for critical information.
10. D.C. bus service is excellent and generally reliable. See the WMATA Metrobus site for information. I recommend the 42 bus, "S" buses, 30 buses, L buses. You can also use the Circulator [tourist] bus service to get around major areas around town. Both MetroBus and the Circulator buses take SmarTrip® cards. The Circulator is a non-Metro service but stops are side-by-side. Drivers can be grumpy and disinterested.
11. Taxis are uneven in quality but are ubiquitous. Lines at the airports and Union station are very long - don't be in a hurry if you end up in one. Drivers can be grumpy and disinterested.
12. There is also the Uber on-demand limo service - you use a Smartphone app to hail one using an account you set up.
13. Train announcements can be hard-to-hear and the audio system is not-so-great. operators can be grumpy and disinterested. They have little system maps available at station kiosks. It's a pretty easy system to get around. If you get on the wrong train (hard to do), get off and simply go over to the other platform.