A button behind a pane of glass. Pressing the button opens a hatch in the ceiling from which water runs, the Redstone torch flashes and you hear an alarm sound.
Try to recreate the redstone circuit from the picture.
On the top left of the screen is a memory cell to store whether the alarm is currently active or not. At the top right is a pulse generator which triggers the alarm redstone flare and the note block.
To reset the alarm, simply reset the memory cell, e.g. by means of a button, and replace the glass pane.