1. Go to your WordPress dashboard. Click on Settings > Discussion. Un-check the Allow people to post comments on new articles. This will disable the comments option on a new article.
2. For the old articles, you may need to check individually. Change the post to change/edit mode. Find the Discussion tab (if not there, go to Screen Options, tick the Discussion field box). Then, un-check the Allow comments option.
3. You can also use Disable Comments plugin to disable globally the comments option on your WordPress. This plugin allows administrators to globally disable comments on any post type (posts, pages, attachments, etc.) so that these settings cannot be overridden for individual posts. It also removes all comment-related fields from edit and quick-edit screens. On multisite installations, it can be used to disable comments on the entire network.