Shutter does not take a pictur in AF mode

Try removing the lens with the camera off and re-mounting it. Before doing so, check to make sure the contacts on the lens and camera body are clean and not damaged. If that doesn't help try gently pressing on the front of the lens (not the glass) while pushing the shutter release button half way down and if it works then you've got a poor fit between the lens and camera body.

The AV mode is for setting the aperture (ie Depth of Field).
To change the ISO setting turn the Mode dial to the P position and then turn the Main dial (next to the shutter button) to the ISO setting you require.

Try this...remove the lens and fire off a few shots. Clean the contacts on the lens and camera body where they meet with a cotton swab lightly dampened with rubbing alcohol. Re-attach the lens and try it.

Will it not pop up under ANY circumstances? Try putting the camera in Manual (M) mode, set the shutter speed to 1/200 and the aperture to F5.0, then go in a dim room. Press the button on the camera body with the flash symbol on it. Does it still not come up?

Mirror lockup is probably enabled. You can disable it by going into Menu and then Custom Functions. Select Mirror lockup and disable it.

Most entry level SLR cameras will struggle to focus in low light, especially with the kit lens. By rights the flash should popup and emit a flash pulse to help focus. This is called focus assist. You might have to consult the manual to check if this function is turned on or not.

It could well be the lens, Take it off and clean the contacts on the lens and body,

shutter struck, if the battery has no enough charge the shutter motor will not rotate fully,charge the battery and try.

To NOT get that error you need to use in AV mode. It's the only way that old Sigmas work with the new Canons. It's an aperture bug that old Sigma lens have and some the only way to fix it is to rechip it. The problem is that Sigma no longer rechips the lens.

You will need to read the manual. I f you have a basic understanding of how ISO, shutterspeed & Aperture width combine to determine the 'right exposure.

Try this:
Set the camera to Av (aperture priority mode). Half press the shutter button and see the light meter indicator to see how well exposed your shot will be. For most cases you would be aiming for a value of 0 (properly exposed). The metering mode determines what part of the frame is used to compute the correct amount of light. For starters begin with pattern metering. Try and aim for a shutter speed of 1/125 or more if you are using the 17-55 mm EF-S f3/5-f5.6. Try and shoot at 40 mm F/5.6 (in Av mode this can be set by rotating the dial near the shutter button.
All the best