How do I upload products to my order form?

Do you mean that you are providing users some digital products after the purchase / payment?

If so that would need to be shared over thank you page.

If you however mean that you would like to add the products to your form, I would suggest checking out the following guide: Setting Up Your First Order Form

It will show you how to add products to your order form and few other small details that will help you set up your first form.