Tax on Amazon KDP earnings

I just published a book on Kindle Unlimited/Amazon KDP and it's gaining some traction and I expect to earn some income from it. I am also starting a full-time job soon. How do I report my income from book sales? The income will be varied every month because it depends on how many people read the book so there is no fixed income. I am not sure how to report/pay taxes. I just graduated recently and I am entering the work force now so I don't really know how this works 🥲

Thank you in advance for your help 🫶🏻

by Aggressive_Bowl5463