How to make a user guide of an application ?

Dear Friends:

In my application, there is a Help menu. Under the help menu, there is a submenu called User Guide. When user clicks the User Guide, he can get information about how to use my application. In Technical part(VC++), How do I make this User Guide text file and linked it to the application? Do I need to write this file in WORD or HTML or another tool ?

Thanks.