When do i have to start paying my phone contract?

Hi I'm getting an iphone 7 soon and i'm buying it in monthly instalments (its not a contract but its just for the phone) of 15 pounds a month. Do i start paying this as soon as i purchase it or after 1 month?

Right away

If you're paying 15 quids per month, you've to pay 15 every month and it will start from next bill.