Lisa, put your ISO at 400, and set your mode dial to "P". On the side of the lens, set your stabilizer to "on". Twist the zoom ring on the lens so that it is at the widest (18mm) setting. Now point your camera to some bright area of the room and half-depress the shutter. The camera will show you the shutter speed and the aperture it "thinks" you need to get a good exposure. Let's assume it gave you a reading that looked something like this: [ 1/4 ][ 3.5 ]. This means it selected 1/4 second at f/3.5. That aperture is the widest your lens is capable of, and it is in effect "wide open". The 1/4 sec. shutter speed means you will need to hold the shutter open for 1/4 second to get enough light to have a "normal" exposure.
As Sparky mentioned, that's very slow and you will probably move the camera a bit when you release the shutter. Usually you want the shutter to be above 1/45 to 1/60 sec to avoid hand-held blur. Plainly put, there's not enough light to get a blur-free image while hand-holding. but take the picture anyways. You'll use it as a reference.
What to do? You can add more light. You can increase the lens opening and let in more light (not an option with your lens, as it is already as wide open as it can get). You can boost the sensor amplification (that is, raise the ISO) so that the sensor can "see" more light - but doing that also amplifies any noise so your picture becomes "grainy". Just for giggles, change the ISO to 1600 (which is 2 stops-equivalent more light than ISO 400). Now if you aim at the same scene you shot earlier, and half-press the shutter, the reading should now be [ 1/15 ][ 3.5 ]. The camera has increased the shutter speed to 1/15 sec at wide-open aperture. Take another picture. When you download it to the computer and look at it using the DPP software supplied with the camera, you'll see that is probably with lots of "grain", but the image may be sharper.
OK. Now change the mode dial to Av (which stands for aperture value), and rotate the dial behind the shutter so that the aperture reads [3.5]. If you half-press the shutter, you'll see that the camera has chosen the same 1/15 sec value as before. That's because as long as you are pointing at the same scene, and you select the same aperture and ISO, the camera will pick the shutter speed that allows a "correct" exposure, which was 1/15 sec.
Try playing with the "P" mode to see what the cameras recommends as the "right" exposure, and then try and duplicate the same using the Av mode.