First, go to Disqus.com and create an account with them. After that to create comment system with Disqus on Wordpress, just go to a left panel of your Wordpress admin, select Plugins > Add New.
From plugin search box--search for "Disqus" and find the plugin called Disqus
Select Install Now > Activate Plugin.
Proceed with the onscreen install instructions.
Log into your Disqus account, then choose the forum shortname you would like to install.