How can I create a new Fan custom tab?

I believe you are looking for how to create a LikeGate. This is a function of an application that stops the user from further action until they have Liked the Page. To create one, you have to set up an application. The code used for these pages is fairly simple, and you should be able to find an example that you can use online.

A great resource for LikeGates: http://www.sayitsocial.com/blog/2010/12/how-to-like-gate-your-facebook-page/

And that is How can I create a new Fan custom tab!