How to use CL programming?

I want to ask about the CL programming and As400. I’m new user of Cl and I want to create a new program in CL using commands. So what is called menu and where from I need to offer the values which is exact according to the application? And where I have to store the database files in whether in the As400 and direct in CL commands?
Please suggest me the best method that will help me very much..
Thanks
