It’s a heart sinking feeling when you realize that your iPhone is gone—left on the back seat of a taxi, on the table at the restaurant, or stolen from your hand bag. You may not get it returned but there are steps to take to protect yourself and your data.
First, you should have your iPhone set for automatic backup to iCloud. You do this in Settings -> iCloud -> Turn on iCloud Backup. Also, turn on Find My Phone.
These two steps assure that you have a copy of all your phone data so that it can be restored to a new iPhone, if need be. And you can prevent others from accessing your data.
When you discover that your iPhone is missing, immediately go to iCloud.com and login with your Apple ID. Go to Find My Phone and put your phone into Lost mode. You have a few options. The first option is to lock the phone and display a message for the person who finds it to contact you. This works well if an honest person finds the phone. If it has been stolen, or if no one contacts you, you can take the next step and fully erase the phone of all your data. Find my Phone will also show you the location of your iPhone. Unless you recognize the location, for example, your doctor’s office, the cafe where you had lunch, it’s probably a bad idea to go knock on a strangers door and demand your phone be returned.
You can read more about locking and erasing your iPhone here.