Hey, I am writing a game and i want it to pause every time my mouse leaves the window and everytime i have another window on top of it, how do i do this?

Have a global flag for "gamePaused", run your game loop whenever the flag isn't set. To set/unset the flag, hook the WM_ACTIVATE message in your Windows loop, setting your paused flag if the WPARAM is set to WA_INACTIVE and unsetting it if WPARAM is WA_CLICKACTIVE || WA_ACTIVE

This will pause your game whenever the game window isn't active and restart it when it is.