You can write a lot of code, buy 3rd party libraries, or use an application intended for the purpose. You probably won't find everything in one neat package as free VB6 code or even in a single library.
Reliably getting webcam images with VB6 usually involves DirectShow, and even then you'll need 3rd party library support in most cases though you can achieve very basic results without them.
Then once you can capture frames you'll have to "find" the QR code within them, isolate that part of the image, detect and correct for keystoning, enhance the contrast, and then decode the barcode. I don't know of any free copy/paste code for that but you might find it if you search hard.