We are flying into BWI and would like to know how far to Washington DC and what is the best transportation to get there

If you rent a car you want to get on Baltimore Washington parkway south route 295 will take you straight to DC. If you are taking a cab take one of the BWI cabs they can take you right where you need to go from BWI its about 20 miles to DC.

There is a train you can catch from the airport. Its a little over an hour. Uber is around 60$ and it takes about 45-90 minutes depending on traffic. I have done both from Reagan to Baltimore, and from Baltimore to Reagan.