There are a LOT of places to learn about linux. Where to look for information largely depends on exactly what information you're looking for.
Experience is probably the best teacher - just use linux, find ways to do things other than clicking icons. Edit config files, use the command line, if you get into trouble or don't know what to do, google and the forums have a huge amount of information available to those who know what they're looking for.
On ubuntu a good place to start might be the docs. https://help.ubuntu.com/
As for helping skeptics - just be available to help install and run, show them how to do whatever they need to do, and try not to get aggressive about getting people to convert. If they don't want to use linux, you can't make them.
Another recommendation I have, once you get comfortable with the command line, give Slackware a try. It's not exactly friendly to inexperienced users, but to do anything on slackware requires you to know a bit about linux. I've learned more using Slackware than I ever did on other distros I used.
Oh, and podcasts are your friend: Going linux, The GNU world order, Linux Realities(old but still useful), and many other pod/ogg/audio-casts exist with linux as the topic, and you can pick up quite a bit of information from them.