To find the registrar of your domain name, go to one of these recommend lookup websites:
Enter in your Domain (ie. iheartsitebuilder.com) and you will be shown information regarding your account. You can view the IP Address and your DNS hosting provider.
The WHOIS database contains public contact information for all domains on the internet. In the list of WHOIS information, you will find a line titled, “domain registrar”, or similar.