If you want to start a thread (a series of replies to an original post) then just find the best-fitting forum for your post, and hit the new topic button, just as you did to make your introduction.
BC has a forum that explains how to do many of the common activities here; you might want to take a few minutes and read some of the guides, especially if you are completely new to forums:http://www.bleepingcomputer.com/forums/f/82/new-user-orientation/
There is even a forum where you can practice or test something:
"Tests and Scribbles", at the very bottom of the forums.
Welcome to BC!