To reply to a topic or any replies already posted, you have two options.  Toward the bottom of the page is a "Quick Reply" window.  Just type in your reply and sned.

The other option is to click the "Post Reply" link.  This option allows you to apply some basic formatting to your reply.

Note that anytime you are reading a post, you can create a new topic within the forum you are in.

For more help, click the "Help" link at the top of the page.