You can start a discussion by simply leaving a comment on a page, like this one.

Why not give it a try?

Go to the bottom of this page and start typing in the comment area. When you're finished just press save! 

Don't just confine your comments to the bottom of the page - highlight some text on the page to add an inline comment like this:

Hint: You can mention another user in a page or  comment by typing @ and then the user's name. 
The user will be notified that you mentioned them.