How does the monthly pay works for an iPhone 13?

Hi there, I'm looking to pay monthly for an iPhone 13 and was wondering how the pay works once I decide. The phone I would like is £36.62 per month due to the 256GB storage on the phone. I'm not sure if I get the phone and pay monthly after that, or if I have to pay a certain amount upfront then pay monthly. And if I do need to pay before my monthly payments, can I please get a price on how much it would be

[Re-Titled by Moderator]