You need to go to your domain registrar. You have to purchase a domain name from a registrar. After the purchase is complete, it should ask you how to configure that domain. You can give it the "A Record" of 198.91.81.15.
If you provide your domain name or the name of your registrar, I can provide more precise information.
If this is not a TLD (like yoursite.example) and is a subdomain (yoursite.sometld.example), then you will have to provide an A record as well. You can also post that information if that's the case.