This is a good overview of some things
and then there is here
http://www.reddit.com/help/commentingIf you don't know how to find the mantic area just type mantic into the search bar. The big blue link won't say mantic, but in the bottom right corner of the info bar there will be the name of the 'subreddit'. Click on that and you should be in the mantic subreddit.
There is a green button on the right. Click subscribe.
You can search pretty much ANYTHING you're interested in.
I'm on a fallout subreddit, Babylon 5, Religious(Baha'i), science, worldnews, for hire and so on.
