Try the web site of the Federation of American Scientists. In addition to many essays and articles about this topics, they have a variety of interacvtive pages and games that allow you to experiment with the subject.
Examples
The Blast Calculator: Drop a nuke of a size you select on the city of your choice (well, simulate it, of course, don't really drop anything). This will show you where the damage is.
The Fallout Calculator: Check the zone in which radioactive fallout woould be a problem in circumstances similar to the blast calculator above.
The Uranium Enrichment Guide: Uranium must be enriched in order to make nuclear weapons out of it. Enrichment means increasing the percentage of the explosive variety of uranium (Uranium-235) in the sample of uranium that you have. This step is the most difficult one, and it is the big roadblock for nations, terrorists, gangs, your neighbors, and anyone else wanting a nuclear weapon. The favored way of doing it is with a whole bunch of centrifuges. Explore how long it might take under various conditions.
What About the North Korean Program?
Try this Public Broadcasting System (PBS) Link about the Feb., 2007 agreement in which Norfth Korea agreed to give up its nuclear prograam in exchange for energy aid. It contains many links about the subject.
Also try this link to a Federation of American Scientists page giving the history of the North Korean program using a brief synopsis and many links. It is up to date through the Oct., 2006 North Korean nuclear test (as of March 11, 2007).