A domain name is the bit of the URL (the site address in your browser’s search bar) that identifies a web page — in this case your website. For example, ours is websitebuilderexpert.com. You can register them separately at sites like GoDaddy and Domain.com, but website builders offer to do it for you when you sign up with them. Most provide it for free (at least initially), while a handful charge a few extra bucks.
If you prefer a more traditional URL, you'll need to purchase one from the likes of GoDaddy or Namecheap. Domain name pricing can range from extremely cheap to extremely expensive, depending on whether or not domain squatters are looking to flip a valuable piece of online real estate. You'll want to get something short but evocative and catchy, and depending on what you do, you may find that many of your first choices are taken by either other legit domains, or by squatters who've scooped up the names as an investment. For more, please read How to Register a Domain Name.
After you choose a website builder and a theme, take some time to learn how to use and customize these tools. Once you feel comfortable, start making changes. Some easy changes to start with are to add your name, logo, and contact details. Then, change the text font and color, add image galleries, background pictures, banners, and other site enhancements.