Go to the bottom of any post that you want to reply to and you will see a "Leave a Reply" section.  If you are not logged in then it will state:

You must be logged in to post a comment.

Click the "logged in" link and it will take you to the WordPress logon page for my blog.  If you aren't already registered then click the "Register" link to create your own user name.  A password will be mailed to the  email address you provide.  Once you get that email then you can log on with your ID and new password and reply to your hearts content! :)

Give it a try!

Its fun! :)