For admins and end users, when you sign in to https://portal.office.com, you will go to the Office 365 Welcome page. The tiles that appear on the Welcome page depend on what licenses you have. For example, if you have licenses for Office 365 and Dynamics 365 (online), you'll see tiles for Microsoft Office apps like Word, OneDrive, and SharePoint, as well as a tile for Dynamics 365.
Click the Dynamics 365 tile to go to the Dynamics 365 Home page. See For admins and end users: Introducing the Dynamics 365 home page.
If you're a Dynamics 365 system administrator, you'll see an Admin tile. Click this tile to get to the Office 365 admin center, where you can see your service health, manage users, manage licenses, and more for all the online services associated with your account.
From there, you can get to the Dynamics 365 Administration Center. Click Admin centers > Dynamics 365.
For other ways to access Dynamics and Office 365 apps, see Access Dynamics and Office 365 apps in Dynamics 365 (online).