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Hi Aneela,
I'm not sure I can wholly endorse La Pistola's comments.
Quality does not come from long exposures and wide apertures.
The first thing you need to understand is the triangle of exposure. The objective is to get the right amount of light on the sensor to give you the correct exposure. This is done by juggling the shutter speed with the aperture setting. Higher speeds will allow less light in that lower speeds and this is offset by using larger or smaller apertures. This is further adjusted by use of the ISO settings; This effectively adjusts the sensitivity of the sensor and lower numbers need longer shutter speeds or larger apertures.
The higher the ISO setting the greater the chance of noise interfering with your images, so you should try to use as low a setting as your type of photography will allow.
A very basic rule of thumb might be: Outdoors, bright sunshine, fairly static subject. ISO 200 with a shutter speed of 1/200 @ f11 Check out the histogram and adjust any one of these as necessary to get a good spread over the graph.
The next thing to consider is the optical performance characteristics of your lens. Every lens has a 'sweet spot', that aperture setting where the lens will produce the sharpest image. It will not be at either end of the scale, so try to avoid the widest couple of apertures and the smallest two - the rest should be fine.
Now, your shutter speed. This is dictated by your subject. If you're shooting racing cars or low-flying aircraft, you're going to need a fast shutter speed. Perhaps something around 1/500 and adjust your aperture to suit.
However, if you find the light is bad but you still need a fast shutter speed and you haven't got a large enough aperture, you will have to increase your ISO setting to compensate.
OK. So, here are the rules. ISO 400 lets in half as much light as ISO 200 and ISO 100 twice as much. 1/500 lets in half as much as 1/250 and 1/125 twice as much.
When it comes to apertures it starts to get a bit tricky. The theoretical widest possible aperture is f1, f1.4 lets in half as much light as that and f2 a quarter as much. These are referred to as 'Stops' and they run (rather bizarrely) f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32. Each of these steps means half as much light as the one before and needs a corresponding increase in shutter speed (or ISO) to compensate.
I hope this has made it a little clearer. It takes a little while to get your head round it, but it will be worth the effort in future as this is the basic photographic principals that it's all built on. Feel free to continue to post any questions you may have, even if it's just to say that all this makes no sense. We're here to help.
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