Hi Eric,
Thank you for your post here.
After we getting start with Office 365,The defualt Websit URL includes ".sharepoint" designation.However,we can add a own domain and then change the Website name.Please refer following steps.
1.Add a domain.
To do so,please see Add a new domain to Office 365
2.After you add your domain to Office 365, you can change the default domain for new email addresses. For more information, see Use a custom domain name for your SharePoint Online public-facing website address.
Thanks,
Keats Ge