trying to make vm show message instead of making payment when checkout clicked

[shenkwen]

Hi everyone,
    I'm very new to VM, am using VM2.0.6 now.
    I want to show a customizable message to customers after they checkout, instead of going to payment pages. I understand that there maybe no plugin or extensions that can achieve this, so I am trying to modify PHP files. Although I have very limited PHP experience, it seems not very hard.
    I think to do this, I must modify the file which is pointed to by "action" attribute of the form where "checkout" button is in. Now by inspecting the page with firebug, the action attribute is like this:
    action="index.php/component/virtuemart/cart/checkout?Itemid=0"
    How do I know which file this is?