There are a variety of resources available to help you learn about the NIEM. Here are just a few:

NIEM.GOV is the official home of the NIEM. It's the main stop for the latest NIEM news as well as access to the official distributions and many of the tools.
NIEMTRAINING.ORG contains a myriad of links to NIEM training resources.
NIEMTOOLS.ORG is the home for many tools we've created, in conjunction with the NCSC, as well as links to other NIEM-specific tools. Now technically superseded by the NIEM Tools Catalog.
NIEM Wayfarer
NIEM Wayfarer is an online exploration tool for learning about the NIEM and how it's put together.