Unless you are a competent welder then you should either take it to a professional exhaust shop or just replace it. If you can weld thin you should be able to remove the rusted section and the weld in a new panel. However you need to know how to curve the patch to fit and how to remove the chrome.
The best solution would be to weld the pipe shut, but if it is very small, you can do this with a little soldering iron and some heavy-duty solder. Be sure to wear protective glasses when using these kinds of tools, and if you are unsure, go ahead and take it to the mechanic.