I did post earlier, but you didn't respond so I don't know if it was stuff you already know or not.
TV mode will let you set a shutter speed that you require, the camera will then adjust the aperture value to ensure that the shutter speed is what you set it as.
AV mode will let you set an aperture value that you require, the camera will then adjust the shutter speed to ensure that the aperture value is what you set it as.
Together with the two above modes and also the manual mode, you can use Auto ISO. This will change the ISO value to ensure that the shutter\aperture values that you set can be achieved. Obviously if lighting conditions are not suitable for the values that you are using the camera may not be able to set the values automatically.
I would recommend that you spend some time in manual mode with Auto ISO enabled. This will let you mess around with shutter speed and aperture values to see the results.