Q: How do I turn off the automatic renewal data plan for the iPad?

Go to Settings => Cellular Data => View Account, logon with the email address and password you used to setup the service, then tap Add Data or Change Plan => Cancel Plan.

I would imagine before. Once it renews, it's paid, you bought it, and the majority of vendors will NOT offer a refund, or will make it difficult.

Thx, but how do I retreive the e-mail address and password that I initially used?

Contact the provider you set those up with. I suspect go to their website, or contact their customer service.