Use the arrow keys to move around. The red bar on the left and the red bar on the red blood cell displays the same health, and it is the health of the red blood cell. 
Don't let the red blood cell run out of health. The yellow bar on the left is your stamina. You use stamina by hitting enemies or using your ability. 
If you click once, you can see a line from the player to the mouse, and if you click again, you get launched in the direction of the yellow line. 
You aim with your mouse.