Show a required textbox when a variant is selected

[booknerd]

I'm using the Studio theme

329543-2.myshopify.com/  (DNS settings are updating, so the page is down at the moment).

I have a product with two variants. What I'm looking for is when 1 of the variants is selected, a text box appears so the customer can customize the product. I would like the text box to be required before they can add it to cart. I'd also like whatever custom text they enter to show up on the shopping cart page.

Thanks!