If I write an application in Excel that has commercial value, and I intend to sell it, where do I stand on the copyright issue?

I assume I automatically have the copyright on the application, because I wrote it.

But what happens if someone steals the code, or just puts their name on the app., and then sells it for their own profit?

In other words......even though I have copyright on the app., how do I prove this? Do I need to register it somewhere or something?

What do you guys normally do?

Cheers
-Rob