Best way to integrate different product sizes (S-XXL)

I am working on building a small t shirt shop, but I am wondering if there was a step I was missing where a person could choose different variations of a product within the same product? (If that makes sense). My site is dreamsuniversity.com so you can see the t shirt i set up on the home page. I just wanted the customer to easily choose a size from the beginning. Suggestions?

Also, I put in the paypal api password/signature/ etc but it seems to not be hooked up with PayPal yet. Suggestions?

Thanks, guys. :slight_smile: