As modern technology and sensor development improves, higher ISO’s can be used with less of a detrimental effect on the image.
Shutter Speed is how fast the shutter opens and closes in a camera or lens, controlling how long light is exposed onto a sensor for. The main deciding factor for shutter speed should be how much your subject is moving and at what speed (and how well lit the subject is). A faster shutter speed (eg. 1/2000) would be used for sports or wildlife as it will freeze the subject and everything else in the frame. A slower shutter speed (eg. 1/15) would be used to create motion blur and will allow for some movement in the image (think a waterfall with smooth cascading water).