how do i download the app store?

Hello,

You cannot download the Appstore as it is a default app that comes with the iOS and through which you install other apps.

But, for some reasons if you are missing Appstore, try searching your device cause it might be grouped together with other apps in a folder. Pull down from top of the home screen to search for Apps.

If it does not help, install the latest OS using iTunes and it should get you all the default apps.

https://support.apple.com/en-nz/HT204204