How do I use Bitcoin as a regular payment method or an alternative to Paypal/credit card

I'm a Bitcoin beginner.

Let's say I am on an online shopping website for clothes, and there is a payment method for Bitcoin. When I click the link it goes to a page which presents a QR code. I've scanned it with my CoPay account but it wants me to add funds which I have no idea how to as this presents me with another QR code to scan.

Are there basic step by step guides to using Bitcoin as a payment method like Paypal for physical and virtual goods?