Finding the right ISO
One of the greatest advantages that digital cameras have over film is the ability to change your ISO on the fly. If you are not familiar with what the ISO is just think of it as the film speed. (In countries outside of the US you may know this as the ASA)
Being able to adjust your ISO between frames means that you can walk into a dark room and take a shot at 1600 then instantly walk outside and take another shot at 200 all without wasting any film. But if you never really were a camera geek in the film days you may not know what ISO to choose and when.
As a general rule I set my ISO to 200 in full daylight, 400 if it's overcast, 800 inside with the lights on and 1600 inside if the room is dimly lit. Don't take that as a concrete rule but as a guideline to get you close to the ISO you need for your photos. You can adjust it from there depending on your metering and your test results.
To really nail down your ISO set your shutter speed to 1/60th (it should read 60 on your screen) and adjust your ISO until your camera's meter is close to zero. That will be the highest ISO you should shoot, anything higher may cause motion blur because you will need a slower shutter speed to get the correct exposure.
Just remember that the larger the ISO number the lower it is considered to be. Meaning that 200 ISO is higher than 1600 ISO. It's a little confusing at first but the more you play with it the easier it will become.