Long story short: if you see a flash, immediately "duck and cover", and get to a sturdy place inside the house, especially away from any windows.
But to your exact question:
If it's a low Hiroshima-yield weapon, and you're a full 15 miles (24km) away, you'll feel warmth like somebody pointed a bunch of heat lamps at you and turned them on for a few seconds, but it's just that. And *at that distance* and *that yield*, you won't get enough radiation to be of any significance.
That's because the intensity of heat and radiation drops off *very* significantly with distance:
http://www.stat.ucla.edu/~dinov/courses_students.dir/data.dir/AtomicBombSurvivorsData.htm#faq2
As to fallout: again, *that* yield and at *that* distance, fallout is probably not a problem, but if you're *absurdly* unlucky, who knows, maybe it all somehow wafts totally and exactly in your direction. So I would hunker down for a while, to let the short-term fallout settle, and await further instructions.
But if NK has managed to produce an ICBM that can reach DC (as opposed to just *putting* one there somehow), they'll probably have managed something much higher than a Hiroshima-strength weapon to put inside it! How strong? Who knows. And the shock-wave becomes a factor. Thus: duck and cover, and get to someplace sturdy and not next to glass.
And now you can stop worrying about this issue, and consciously forget this advice, because it's now in your long-term-storage memory, and will pop up as need be.
I say this from my experience in a big earthquake: I never went around saying "remember: get to a doorframe and close the door and huddle!", but when a big earthquake did happen, I did it before I even realized I was doing it. "Next thing I knew, I was suddenly there."