"To authenticate and communicate with Google Fit, you must create a project in the Google Developers Console, activate the Fitness API, create an OAuth 2.0 client ID, and register the public certificate from your app's signed APK."
What are all these things? I have no public certificate from my app's signed APK, I'm stil debugging on my devices. Do I have to pay a registration fee? If I click on Developers Console they ask me.
You don't need to pay the registration fee unless you actually plan to publish your app in the play store.
Just actually follow the steps as described in the guide that you linked to... It tells you how to obtain the fingerprint of the debug key using keytool.
Follow the links in the guide to the "Google Developers Console" (and not the ones displayed in the top right corner of developer.android.com, that take you to the "Google Play Developers Console" - I admit that's confusing...).