In Av mode you set the aperture and the camera will set the speed and ISO to give correct exposure. If you want to control both switch to manual mode M. Be wary though because Canon cameras will still set the ISO based on your choice of aperture and shutter speed for full manual you will need to fix the ISO as well. The three things, aperture, shutter speed and ISO control the exposure. Look up "exposure triangle" for more info. (Apologies if you know all this already!)
J