There are two ways to backup your iPhone, with iCloud using your personal Apple ID or on a computer with iTunes.
How to back up with iCloud
Make sure your device is connected to a Wi-Fi network.
Go to Settings > [your name], and tap iCloud. If you’re using iOS 10.2 or earlier, go to Settings, then scroll down and tap iCloud.
Tap iCloud Backup. If you're using iOS 10.2 or earlier, tap Backup. Make sure that iCloud Backup is turned on.
Tap Back Up Now. Once your backup is complete you'll see the date and time of your last backup.
iCloud will now automatically backup every 24 hours whenever it is charging, connected to a Wi-Fi network, and has enough space.
NOTE iCloud only gives you 5GB of space to backup. If you need more you can either purchase more storage or choose the alternative backup method.
How to back up with iTunes
Open iTunes and connect your device to your computer.
If a message asks for your device passcode or to Trust This Computer, follow the onscreen steps.
Select your iPhone, iPad, or iPod when it appears in iTunes.
Just click Back Up Now.
When the process ends, you can see if the backup finished successfully on the Summary screen in iTunes. Just look at Latest Backup to find the date and time.
