If you have an iPhone 5s or later, then the Home button has a built-in fingerprint sensor that can be used to unlock your iPhone or purchase apps and songs. If you have an iPhone X, instead see our guide to setting up Face ID.
To control this, open Settings, then select Touch ID & Passcode. Enter your passcode and from here you can control if your fingerprint can be used to unlock your iPhone or purchase items from the iTunes Store and App Store.
Below this is the list of currently registered fingerprints. You can save up to five. To make a new one, tap Add a Fingerprint and follow the steps.
If you touch the Home button while looking at this list, the relevant entry will flash. To remove one, tap the entry then tap Delete Fingerprint. You can also tap the name and change the name of the fingerprint if you want to.