Getting the contact info

Getting the owner(s) info: Most of the time, these sites have some kind of contact form or their email in the footer of the site (at the bottom).

Getting the host/registrar info: You can use a WHOIS service like this one to get info about the registrar/host (preferably look for the hosts details). After you've found the name of the host, you can look it up. Most of the time they have an abuse form or an abuse email.