How do we put together a shopping cart?

how do we put together a shopping cart

I believe you are planning to create an order form where your users can select a product and pay it online. I recommend you check the following guides:

Order-Form-Basics
Order-Form-Types
Setting-Up-Your-First-Order-Form
How-to-Integrate-Form-with-Paypal-Standard
Mastering-Payment-Form-Integrations-with-JotForm

Hope that helps. Thank you.