If you fancy trying your hand at creating a brand new top-level domain for people to use, the first step is to apply to ICANN. If your registration is successful, you’ll become a registrar and have the ability to decide who can use their domain on your TLD.
Some TLD applications can be quite contentious. For example, Amazon has applied for .amazon and have waited years for a decision over their registrar status. ICANN has been dragging their feet due to concerns from countries with Amazonian regions, fearing specific .amazon domain names could cause confusion or touch on national sensitivities.