If you want your customers to agree to your terms and conditions before they complete checkout, you can easily add a checkbox to your checkout page by following the steps below:

1. You must first enable the Checkout feature under Configuration > General.
2. From your Storefront admin's Configuration > Checkout menu, select the Custom field tab.
3. Choose "Basic" for the dynamic form dropdown.
4. Click Add new.
5. In the Field type, select "CheckBox".
6. Give the ID a name like "AgreementCheckBox" without spaces.
7. Give the Label a title like "I agree to the terms and conditions:".
8. Check the Required checkbox.
9. In the Validator text, enter an error text like "You must agree to proceed."
10. Click OK.
11. Click Save.

If you want complete customization over the look and feel of the checkbox and text, you can create the custom field following the steps outlined above and then modify its design and behavior through custom Display Templates.