So you need to know who is hosting a website?

Generally, it's easy. Simply go to whois.domaintools.com and add the domain name after the slash, like this whois.domaintools.com/mydomain.com

Look down the list, and you'll see the registrar where the domain name is located, and also the name servers. The name servers are what point to the hosting provider. Sometimes it will be the name of the hosting company itself; in some cases it may not be, so you'll have to do a Google search to see who owns those domain servers.