2. For a Newbie-friendly gentle introduction, Boundless Geo offers an online workshop called Introduction to GeoServer. The workshop covers everything from how to install the entire Open Geo Suite software stack (GeoServer, PostGIS, GeoExplorer, and GeoWebcache) to how to load, publish, style, and share geospatial data, with an emphasis on sharing data to Google Earth.
4. For those who prefer to stick your hands right in all the muck and goo, the Penn State GEOG 585: Open Web Mapping Course offers a more standard online classroom learning environment. This 10 week online course features GeoServer amongst other FOSS (free and open source software) including QGIS, GDAL, Open Street Map, & more. You have the option of enrolling and completing the instructor-led portion for graduate-level credit, or for the self-motivated learner, you can simply work your way through the open courseware style class without any accolades.
5. Last but not least, if you're looking to join a more interactive, supportive community where you can join discussions, and post questions and answers about using GeoServer, check out the StackExchange. You can run a quick search for all the questions tagged GeoServer. Here's a snapshot of some of the discussion taking place there right now.
There you have it, I hope these sites help you get started using GeoServer.
P.S. This article wouldn't be complete without a shout out to the #gistribe for all of your great suggestions. Be sure to follow @CartoChris @krmoseley @roleiviraGIS @lbm1982 @mapperz and @ijturton on Twitter for even more FOSS-infused inspiration.