how to tips - Visit Washington DC

The best time of the year is late March and early April (spring season) when there is the Cherry Blossom Festival.
Make sure to use the Uber and/or Lyft (as long as it is up and running). It is very useful for a newbie visiting the capital.
If you are traveling on the weekend, especially if there is an event or holiday, the Uber will charge you a hefty price due to the high traffic and demand in Uber's or Lyft's network.
You may want to fly to the close by airports (e.g. Baltimore) that may save you a lot in tickets.
Hotels are very pricey; pick a hotel close to a metro station and you can save lot that way. Marriott Wardman Park hotel is my favorite (no free wifi and no free breakfast). It is in a walking distance from the metro station, and in the walking distance from the DC Zoo.
Due to the proximity to the Atlantic, a few weeks in the summers can be very hot and humid and a few weeks in the winters can be windy and cold. Check the weather before you travel.
Restaurants are normally packed; especially in the morning for the breakfast. Make sure your hotel has a complimentary breakfast otherwise couple hours may be lost in a day.
Metro trains are fast and available throughout the day. A day pass or a week long pass (~$35) may serve the best for you.
Rental bikes are abundant especially in the National Mall (Smithsonian) area. Make sure to rent one (about ~$8 and hr)
Make sure to have a small backpack for water, wallet/purse, keys, napkins, shades, sun screen, snack, map, and else.

And finally, make sure to visit these attractions:

All the free museums (well US tax payers pay for all that) - last time I counted 19 free museums, my favorite is the National Gallery of Art
US Capital, White House, Washington Monument, Jefferson Monument, Lincoln Monument, National Cathedral (my favorite)
The Georgetown area (M st @ Wisconsin Ave) for Shopping and the crowds
Canoeing or Kayaking in the Potomac River
Strolling/Biking along the potomac river (@Georgetown Waterfront Park)