How I can sapture a picture in ISO mode???
what is the purpose of capturing aphoto in ISO mode??
ISO is not really a "mode", every photo you take has an ISO rating; it's one of the three main ways of controlling how light or dark a photo is. (The other two are shutter speed - how long the sensor is actively capturing for, and aperture - how wide is the hole (aperture) that the light passes through.)
You can think of the ISO rating like a volume control - it's how sensitive the sensor is. At ISO 100 the sensor is at its least sensitive - you get the smoothest picture but you need lots of light to do it. At ISO 800 and higher, your sensor is most sensitive - your camera will take pictures in lower light, but there will be more 'noise' - meaning the speckled effect you get on digital pictures.
It's similar to the volume control on a radio. When you have a strong local station you can turn the volume down, and you'll still get a great quality sound. But if you are trying to pull in a distant station, you turn the volume up and put up with a bit of hiss just to hear what they're saying.
Just the same with a camera. When you have strong sunlight, you can turn the ISO down and you'll get a great picture. But if you're trying to pull in a dark photo, you turn the ISO up and put up with a bit of speckling, just to see the image.
Hope this helps.