MS Teams is now included in the desktop installation package Office 2019 installed as a part of Office 365. If you are a new Office 365 user and you are newly installing the desktop Office from Office 365, MS Teams will be automatically installed as a part of the package.
If you already have Office installed, then you must download the installation package for MS Teams from this page.
For Android and iOS phones, users may download the app from Google Play and the App Store.