Controls are
A for crouching behind your cover
S to get up from crouching position
R to reload your sniper rifle

Whenever the alarm goes off you need to take cover to avoid being shot by the enemy; it takes only one shot to kill you and end the mission in failure. When the alarm stops you can get up and continue eliminating targets.

Targets types and their numbers are listed in the lower right corner of the level screen; these are two types of soldiers (one type of soldier wears a light blue uniform and stands still while the other type wears a light brown uniform and walks around the level) and security cameras.

You need to eliminate all targets before time runs out to avoid faliling the current mission, eliminate all targets in time to advance to the next level.

If you lose a level, click the left button in the game over screen to retry the mission, the right one brings you back to the main menu.