Getting a driver’s license is not a simple process in the U.S. Depending on whether you already have your license in your home country. Read about the process on the ISS website page for driver’s licenses and on this DMV page for internationals. You will need to make sure that if you are required to take the road test, you will need to provide a car for the test. There are also driver’s license workshops you can attend. Check with the ISS office for their schedule.
https://dmv.dc.gov/service/driver-license-for-non-us-citizens
If you buy a car, make sure that you have a parking permit and a driver’s license. Also make sure that you get the title of the car when you purchase it.