I’m Canadian and plan to travel to Washington after the pandemic. I’m eighteen and have never travelled alone before but will have zero guidance from family for this trip due to the fact that they don’t support it because they barely ever let me out of their sight. Never mind let me cross the border. But as I said: I’m 18. They can’t stop me. That’s not my concern. My concern lies in the booking process for flights. I’m most likely going to fly with Air Canada but was wondering what information I would need to book the flight? I kind of figure the obvious full name, date of birth, phone number, stuff. But is a passport number required to book the flight its self or only required at a later time like on the U.S. customs form? Similarly is there any other information I should know about what is needed when booking a flight?

An easy way to find out is to go online & "pretend" to book a flight. Go through the process, stopping just before you'd complete the booking & pay for the flight.

One thing occurs to me, do you have a credit or debit card with which to actually buy your ticket?

Agreed. Best way to learn is go through a booking process, up to the point of needing a credit card for payment.

In my experience you only need the info you listed. birth date, name (that matches what's on your ID), and contact info.
I don't recall needing passport info until time of check-in (online, or at the airport).

I always provide my passport info when I book a flight, but it might be optional, not sure.