The most common domain name tlds used in the world are com, net, and org. However, registering only the .com tld for the domain name you wish to register the domain may create an inconvenient situation for your brand or website in the future. Anyone else can register and use tlds you have not registered, even if this is not intentional, it may be uncomfortable for your brand.
To be a website owner, you must first register a domain name. You can think of a domain name registration as a trademark registration service. The registration service ensures that the domain for which you have purchased the rights for a given period is recorded on your behalf during the period of use and is not used by different people.