Question: how do i download find my iphone app?
Answer: From a Home screen, tap App Store . To install apps, you must sign in with your Apple ID or create one.
Tap the Search (at the bottom).
Enter find my iphone into the search field (at the top).
Tap Find my iPhone.
If presented, tap GET then tap INSTALL. ... 
From the app, enter your Apple ID and password then tap Sign In.