If you survive the initial blast, what will you do? 

The official US government guidance is to shelter in place. You go to the nearest and most protective building and stay there for 24 hours unless told to evacuate sooner.

This isn't bad advice if your immediate shelter is the basement of a more or less intact house, which can reduce radiation levels by a factor of ten or so. However, if the blast occurs in Los Angeles rather than in New York, the lack of a frost line allows most houses to be built without basements. Such houses only block about half the fallout radiation.

Taking into account the decay rate of the fallout, a location with an initial exposure rate of 200 rem/h (about the highest dose rate for fallout from a 5 kt device) will receive a total radiation exposure of over 600 rem in the first 24 hours. If sheltering in a NYC concrete basement, a person's exposure in this period would be about 60 rem, an exposure having little immediate health consequence. However, in an LA ranch house, over 300 rem would be absorbed in that same 24 hour period, which would prove an eventually lethal dose for a substantial number of victims receiving little or no medical care, particularly if combined with flash burns and blast debris injuries. It would appear that sheltering in place is not necessarily the best advice, depending on local circumstances.