how to make a must be first tackled window?

Questions & Answers
1

for example : a windows to tell user whether sure to quit the game.before user clicks the yes or no button , he can't do anything else

can i use gui.window to achieve the goal??

2

Sure. All you need to do is not have any other items on the screen that the user can interact with and you have your modal dialog. And using a Window for that is just fine.

You might also considering setting Time.timeScale to 0 which basically pauses the game.

Scripting reference: Time.timeScale