How do I create chm file type in robohelp?

Provided you only want to generate the CHM and not actually write the content:

1. Open your Single Source Layoits Pod - Click View > Pods > Single Source Layouts
2. Double click on the Microsoft HTML Help.
3. Click Save and Generate

The above asumes you want to use all the default options. If you don't, you'll have to give us more of an idea of what you want to do.