Modifying the User Registration form?

Hi. How can I modify the user registration form? The current form shows Fullname and Email, and the Fullname is then placed in the Firstname field on the user profile page.

I would like to show Firstname, Lastname, and Email. That would make it easier to copy new users over to my ActiveCampaign email list, which captures the same fields.

You can see login page at https://funthingstexas.com/gd-login/ to see what I’m talking about.

Thanks,
John