Discussion is threaded. You may post a response to someone else's message. To see a list of every topic that's been begun, check out the List by Topic link. That will show every top-level topic that's been posted since the beginning of the month. As more people join up and introduce themselves, there's more in that thread for you to read.
Manila takes care of all the drudgery of what things go where.
Yes you can, once you're a member of the site. Start a new topic or click the Discuss links at the bottom of a story or news item. Or, if you see a text box below a message, use that to respond to a message. This Page button for your messages. Same thing with email addresses.