How do i edit what happens when cancel is clicked in an inputbox. NEWBIE NEEDS HELP!

Hi guys,
Title says it all really, i have written a small program as one of my small assignments, i have it all working but there is one thing i'd like to tweak if possible.

I have an input box come up asking for the user to enter their card details and click ok... this then brings a msgbox of "order complete thank you"
HOWEVER...
im getting the same message come up if the cancel button on the input box is pressed....
how could i stop this?

Help MUCH appreciated.

Thankyou in advance

I could use the answer to this too.