how can i customise my own file extension format?

[katy_price]

i have a program i created in c++. its a logon application that i want to use to protect private files.
i would like to know how to create my own file extentions so that only my program can open it.
for example if i double click the file, my application opens and the user logs in, if the correct credentials
are entered they are allowed access to the file (it opens so they can read it).

at the moment my program opens up a word document when the user enters his correct credentials.
i am using the system() function for this. however this file when clicked from the start menu opens up using word.

basically i'm not sure what is best or whats right. is it best to just change the file extension so only my app can open it?
or create and save a document in a custom file format so that only my program can open it and other applications cannot read them?
if the latter, how can i do this, what are the steps?

plus i have looked up on some info online and it says that i have to register my file. does this relate to just
changing the file extension or creating a file format or both? How do i register my own file extension