When it sounds the screen should pop up to two buttons, Snooze to turn it off for 10 minutes and disable which will turn it off until the next scheduled occurrence (it repeating alarm).
If you don't see this screen, slide down the notification bar and click the alarm clock notification and it should bring you to this screen. If, perchance, you have disabled notifications for the clock app, open the app drawer and click on the clock app and it will bring you to this screen.