Just to kind of summarize, there are two "types" of exposure:
1) Irradiated
2) Contaiminated
1) You've been near the source and gotten alpha, beta or gamma ray exposure. The damage is now done, nothing you can really do. If I remember correctly.
2) You've been contaiminated with debris that is radioactive. You can remove most with a shower, and you or someone else won't be "radioactive".
There are a few FEMA Independent Study course available through the Virtual Campus that covers this information. Specifically IS-3 and IS-301.
FEMA On-line Training Website - Main Page FEMA Independent Study Course List