Lin, you are best served leaving the ISO on auto for now. The higher the ISO, the more light the camera captures, but at the expense of quality of shot. It gets grainier and grainer at higher ISO numbers.
A couple days ago I took this photo indoors with the lights off and using no flash. The ISO was set to 6400 and the aperture was at f/1.8 (an extremely wide opening on the lens)
You can see some graininess in the photo but look at how it captured the picture in the dark! That's the power of using a really high ISO.
So if it is dusk and you want to capture that flower image before it gets completely dark, your camera will automatically set the ISO as high as it can go in an effort to get the shot.
