Most of the time you’re going to want to find a .com domain name as that’s the extension most people think of by default.
If you are building a website for a non-profit organization than .org is a good choice.
Regional domains (like .uk and .ru) are good if your target audience is primarily in that region.
In the video linked above we show you how to use Namecheap.com to determine if a domain name is available. We will get our domain for free in the next step so don’t buy your domain with NameCheap.