A domain name is like the street address for your website. When you type in www.google.com, www.yahoo.com or some other address, you are typing in the domain name for that website.
With over 100,000,000 websites on the internet, it may be difficult to get the exact domain name that you want. You have to be creative, use your imagination, and come up with a name that is not taken. There are several sites that can help get the ideas flowing: MakeWords.com, Dotomator, and NameBoy to name a few.
Keep in mind that many, if not most, .com domain names are taken. There are many other domain "extensions" (called Top Level Domains, or TLDs); .net, .org., .info, and .biz are a few examples. Learn more about domain names.