I Need Help With File Association

Hello my friends,
First of all, I'm sorry for my bad English.

I need help with file association.
For example;
There is a text file named "mytest.mtn" on the desktop.
When I double-click this file, my program should open and the text inside the file "mytest.mtn" should be written to the Input object.

For example, when I double-click the file "mytest.txt", the program named "notepad.exe" opens. Then, we display the texts inside the file named "mytest.txt" in the program named "Notepad".
I want to make a program like this.

How can I do this practice?

Can you give me an example of this?
I would be grateful if you could help me with this.

Sincerely yours..