Adding a comment
If you look at the front page of your blog, you'll notice that every post has a link that says Leave a comment (provided that you didn't disable comments entirely). Clicking on that link will take you to the bottom of the post page, which is where comments can be added.
If you're logged into the WordPress dashboard, you'll see your name and a space to write your comment. If you're not logged in, you'll see a comment form that any other visitor will see. This form includes fields to fill in a name, email, website, and, of course, a big box for the comment itself:
Once you type in the required information and click on the Post Comment button, the comment will be entered into the WordPress database. How soon it shows up on the site depends on your discussion settings.