Click on the game 'settings' link in the main menu to alter game parameters. Please note: 'Show Index' displays the angle and power of your shot when you pull back an arrow. 'Critical Hit' doubles the damage if the shooting angle exceeds 45 degrees. 'Vitality' is the number of hits needed to kill an opponent or die.
To shoot an arrow, click anywhere in game window (make sure you have enough room to adjust strength and angle) and move your mouse back and down to adjust the angle and strength of your shot, and finally release the left mouse button when you're ready to shoot.