Check the Domains FAQ if you don't find what you're looking for.
Your initial email address in Office 365 includes .onmicrosoft.com, like email@example.com. You can change it to a friendlier address like firstname.lastname@example.org. You'll need your own domain name, like fourthcoffee.com first. If you already have one, great! If not, you can learn how to buy one from a domain registrar.
When you change your domain's email to come to Office 365, by updating your domain's MX record during setup, ALL email sent to that domain will start coming to Office 365. Make sure you've added users and created mailboxes in Office 365 for everyone who has email on your domain BEFORE you change the MX record. Don't want to move email for everyone on your domain to Office 365? You can take steps to pilot Office 365 with just a few email addresses instead.
Change your email address to use your custom domain using the Office 365 admin center
Sign in to Office 365 with your work or school account.
On the Domains page, choose Add domain.
Follow the steps to confirm that you own your domain and to change your email address.
You’ll be guided to get everything set up correctly with your domain in Office 365.