form or usercontrol ???

Hi,

i am new in c# an i want to write a little program.
My pgm should have a mainform with buttons that open a stockmanagement(3 forms/windows) 
and a suppliermanagement(3 forms/windows).

How can make this ?
Each window as a form or
the mainform as a form and the modules(stock/supplier) as usercontrols or
the mainform as a form and the modules(stock/supplier) as panels ??

I learned the basics from books and the samples and exercises were very simple (max 2 windows/forms).

Can somebody give me links to tuts or samples about my problem.

(sorry for my english, i am from germany and not so fit )

by jogi