Check the Domains FAQ if you don't find what you're looking for.
A domain is a unique name that appears after the @ sign in email addresses, and after www. in web addresses. It typically takes the form of your organization's name and a standard Internet suffix, such as yourbusiness.com or stateuniversity.edu.
Using a custom domain like "email@example.com" with Office 365 can help build credibility and recognition for your brand.
Choose the experience that's best for you:
Buy a new Domain:
Use Office 365 to buy a new domain - we'll set everything up. Learn more.
Use a domain you already own:
If you already own a domain like firstname.lastname@example.org you can add it and start using OneDrive and Office apps right away. You'll need to take a few minutes to setup mail and Skype for Business.
Tip: Not sure where your domain is registered? - Get help finding your domain registrar
Use a default domain for now:
You can use a default domain like yourcompany.onmicrosoft.com to log in for now and add a custom domain later. You can't change or rename your default domain - so make sure it's what you want before moving on.
Call Office 365 Support - Get help setting up a domain with Office 365