If you need an app for the metro I can recommend Metro and Bus
Shows all lines and bus routes, with times, plus there is a metro map if you need it. The metro stations have plenty of signs making it easy to navigate. There is cellular service for the most part underground which is nice too.
Get a smart trip card and make sure you add enough funds to it to get around. If for some reason you cannot exit (you use the card to enter and exit all stations and on all the buses) there are machines you can use to add funds to your card so you can exit the station. Plus the new cards are re-loadable online. If you have a smartrip card laying around check the numbers on the back. If the serial number starts with 0167 then you can reload it online here: https://smartrip.wmata.com/Purchase/AddStoredValue.aspx
Keep in mind there is a fee to get the card, so add 2 dollars on top of whatever you are planning to put on your card.
As for other apps - Guidebook, Lyft, Social Media Apps, Spotify