You can choose to allow or disallow comments on each post or page that you write. To allow comments on a particular post, make sure that the box next to “allow comments on this post” is checked. You can find that box under the main textbox for the post. Here is a screenshot:
This setting may also be set globally by going to Settings>>Discussion and checking or unchecking the “Allow People to Post Comments” box. When you do this, the change will go into effect for all posts made in the future.