Turn off lockscreen notifications- Even if you have a strong password possible, your private data can be revealed by notifications on your lockscreen. This could be appointments, confirmation codes, financial data, etc. The less information that shows up on your screen the better your security will be.
You can adjust this setting by following these steps: “Settings” –> “Passcode” (or “Touch ID and passcode”) –> “Allow access when locked” section.
Turn on two-step verification for both Apple ID and iCloud.
Remember those celebrity leaks from the past couple years? It was because they didn't have two factor authentication on their phones. This feature essentially makes sure that only authorized devices have access to your data.
You can set this requirement by going to https://appleid.apple.com –> “Manage your Apple ID” –> “Password and Security” –> “Two-Step Verification”.
Turn off automatic sync to iCloud
This also part of the reason for those celebrity leaked pictures. Most of them thought that deleting the pictures from their phone was enough, but the pictures had already been backed up to iCloud. Instead it would be wiser to manually sync only the pictures that you want to be backed up.
You can adjust this setting by going to “Settings” –> “iCloud”.
Turn off automatic WiFi connections to known networks
iPhones have a feature that is usually nice to have, which is that it will connect to known hotspots without your permission, like at Starbucks. This is can be nifty, but it can actually be very dangerous because hackers can create their own wifi network with the exact same name and then they can steal data that your iPhone transmits.
You can adjust this setting by adjusting this: Go to “Settings” –> “Wi-Fi” –> “Ask to join networks: on”.
Disable the AutoFill option
If someone got ahold of your phone while it was unlocked, they would be able to get into many websites that you're logged into and take control of it. Not having this option can be inconvenient, but it will be safer.
You can turn off this section by going to “Settings” –> “Safari” –> “General” section –> “Passwords & AutoFill”.