When you first start up your iPhone you will be asked if you want to use this facility, but if you opt-out and later suddenly discover there may be use for this, turning it on is not a problem at all.
You will either have 'find my' as a stand alone app on your home screen (depending on iOS version) or you will have to go through the iCloud.
Follow these simple steps to activate 'find my iPhone' if you are missing it on your home screen:
Start at your Home screen.
Tap Settings > [your name] > iCloud. If you're using iOS 10.2 or earlier, go to Settings > iCloud.
Scroll to the bottom and tap Find My iPhone.
Slide to turn on Find My iPhone and Send Last Location.
If you’re asked to sign in, enter your Apple ID. If you have forgotten this don't panic, Apple will be able to help.
If you have turned this service on and have realised you are missing your beloved iPhone, using the following steps should help to rescue it:
Use the Find My app located on your iPhone to locate and play a sound on a missing iPhone or to locate it on a map.
In the Devices list, tap the device you want to locate.
If the device can be located: It appears on the map so you can see exactly where it is.
If the device can’t be located: You see “No location found” under the device’s name. Under Notifications, turn on Notify When Found. You will then receive a notification once it’s located.