Go to Settings > Security & location. First, tap on "Screen lock" to select the default method to unlock your phone. This can be a PIN or Password or Pattern, all based on your choice. Note: make sure to write it down in case you forget it in the future.
Next, go back to the "Security & location" page and tap "Fingerprint." You will first need to tap your PIN or Password or Pattern, depending on which you previously selected. Next, tap "Add fingerprint" and follow the instructions to press your finger onto the fingerprint reader (on the back of the phone) to properly register one finger at a time.
Once enabled, you can unlock your phone any time by pressing a previously registered finger onto the fingerprint reader on the back of the phone.