Simply type in your website's domain name e.g. www.yourwebsite.com in the top box and your competitions website e.g. www.competition.com (making sure that if you come up on google with the www. enter your domain using that if not leave the www. out, this is necessary as some search engines see www.thiswebsite.com differently to thiswebsite.com) enter your competitors website domain name and click submit. After a few seconds you will get a brief report with recommendations on what you will need to do to beat your competition.
Try our keyword density tool here.