You pay at the beginning when you start to build your site. Different plans offer different features that will affect building your site. For example, CSS customization is only doable with paid plans, and adding plugins are only available with the Business plan. These would affect how you would build the content of your site, and therefore, you would need to pay first before you build.
I hope that helps. If you are uncertain about which plan you should use for your site, make sure to check out the Refund support page, too. You could consider paying for a plan, using it for a little while, and then cancel it if it doesn’t suit you. (Just be careful – domain refunds can only be done within the first 48 hours.)
Let us know if there is anything else we can help you with. All the best!