Installing Unity for beginner

Download Unity Hub from here ​https://unity3d.com/get-unity/download

After installing Unity Hub, open it 
Press "Installs" tab
Press "Add"

Chose Unity 2019.4 to install

In the next step, choose Android Build Support and Press "Done" for further default installation

If you are going to make your app for iOS, then choose iOS Build support

Press "Done " and follow the next steps for installation