How can I log I using a fingerprint?

You must set up Touch ID to read your fingerprint from the Touch ID/Power button in the upper right corner of your keyboard.

Apple provides guidance for adding your fingerprint(s) to the Touch ID system here: Use Touch ID on Mac – Apple Support

Nutshell version:

1. Click the Apple logo  > System Settings...
2. Scroll down and click Touch ID & Password in the sidebar, then click Add Fingerprint.
3. Unlock the panel with your Mac user password
4. Follow the on-screen prompts and place your finger gently on the Touch ID sensor  without pressing down on the button.
5. Continue to follow the prompts and repeatedly lift and rest your finger, slightly changing angles each time until the scan captures the whole fingerprint and its edges.
6. Click Done when you see the message that Touch ID is ready

EDIT - Once you have added your fingerprint to the system, you can use it to unlock your Mac. However, you will still be required to sign in with your password each time you restart your Mac, and at times when you have not used the password for ~48 hours or so.

D.I. Johnson has the info.

Some background on passwords and Touch ID, as the primacy can be confusing here.

Touch ID is a means to enter your login password less often, but does not, cannot, and will not entirely replace the password.

Touch ID is integrated with the Apple Keychain password manager as well, so you can use Touch ID authorize access to your saved passwords for accessing apps and services.

But the password underpins the security of your Mac, while Touch ID is a useful convenience.

From the Mac User Guide:

Use Touch ID on Mac - Apple Support