Help with Facebook Pixel

I wasted quite a lot of money on advertising on Facebook without any benefit, or minimal profit …
Now I started trying to advertise my store with FB PIXEL.
The truth I got entangled, it's like a maze …
Can you recommend how to start learning more about this system before I just waste money?
And I also realized that the Audience tool is also effective.
But I did not understand the effectiveness of the "Audience Size"
Thanks a lot!