Comments can be opened up or closed on posts or pages. This is left up to the individual authors, or defaults back to the site administrators settings.
Leaving Comments on a Post
Under the Title of each post, along with the date, author and Category, you will see a link to that posts Comments.
Clicking this link will send you to the bottom of the post. At the bottom of each post (if comments were enabled by the author) you will find a comment box.
Just click inside of the text box, type your message, insert your links, images, emoticons, etc and then click post comment.
Leaving Comments on a Page
Though Pages do not have a link under the title, they do have comment boxes (if enabled by the author of that page) at the bottom. Scroll down, and you’ll find a Comment Box at the bottom of each page.
Leaving Comments on a Comment
Just look at a comment, under the response, you’ll see the date, and the word Reply. Click that to have your comment sit under the comment you’re replying to.
note: This is also the location of your edit link if you are the owner of that comment)