Before you jump into this, there is something you need to keep in mind. At the moment, this option is only available in build 15042 of Windows 10 (also known as Creators Update); it may well remain in future builds, but it's not guaranteed, and there's no way to backport the option to pre-15042 builds.

With that in mind, follow these steps:

    1.Open up Settings and head to Apps.
    2.In the Apps & features section click the drop down menu beneath the Installing apps heading.
    3.Select the option labeled Allow apps from the Store only.
    4.Close Settings.

That's all there is to it. You can also opt to display a warning about non-Store apps if a user tries to install a classic Win32 app (but the installation will not be blocked) -- just select the Prefer apps from the Store, but allow apps from anywhere option.