You select your URL (website address) when you publish your site. If you publish to a free Yola subdomain, your URL will appear like this: http://sitename.yolasite.com. You can replace "sitename" with text of your choice. For detailed instructions on how to publish your site, visit this link: Publishing Your Site.
If you ever need to check what your website address is, you can go to Settings > Publishing. The website address will be displayed in the "Your Domain(s)" section as the Primary Domain.