How to get a pay-as-you-go iPhone 4s?

I've never had an iPhone before, and I know almost nothing about them, so can someone tell me (in lots of detail) how I could get a pay-as-you-go iPhone 4s? I know that there's a possible way, I just don't know what it is.

As of right now, I own a pay-as-you-go Android Optimus one and I'm with Telus (Canada), in case that helps.

You can use any Telus phone that has a sim card on prepaid plans. Go to your local Telus store and buy a 4S ($649.00 + tax). Take it home and log into your online Telus account. Look for the link to change your device. Follow the instructions.