Selling merch related to a show without copyright issues?

Lets use spongebob as an example. How can I sell spongebob merch without getting copyright issues? If I for example, sold a drawing of his pineapple home on a shirt, could I get in trouble for that? Where is the line drawn in the sand between okay and not okay? Like I'm sure my website couldn't be spongebobmerch.com but would bikinibottommerch.com be okay?