If you want to find out publicly available information about a domain name that has already been registered, you need to use a whois lookup tool.
This will provide you with information about the person who registered the domain. Be aware that some people put down false information in their domain records; despite the fact they are not permitted to do so.
Domain registrars offer whois privacy to customers; which makes finding the true information about the owner of a domain difficult.
A whois lookup tool is a simple script. You will therefore find thousands of websites online that offer this functionality.
Below is a selection of good whois lookup services which I recommend.