ISO setting in a picture taking context affects the light sensitivity of the camera sensor with the best quality at 100 where there is no noise and poorest quality at 12,800 where there is much noise.
Shutter Speed is responsible for controlling the time which light get exposed to the sensor. For a freeze image less light about 1/500 per second and for motion blur image more light about 1 second of exposure.
Aperture referred to as hole size controls the light amounts which hit the sensor. Smaller hole about f/22 implies big field depth and big hole about f/2.8 implies shallow field depth.
All the above three key elements are dependent on each other. For instance, if you create shallow field depth, it means more light hits the sensor and therefore in order to change the light amounts you either adjust ISO or shutter speed.
Since now you have knowledge of the key elements you will come across in your DSLR camera, you can start practicing. For beginners, it’s recommended that you use this settings for a start: Aperture mode as priority mode if using Nikon camera use A and if using Canon camera use AV, for the shutter mode use TV if using Canon camera and S if using Nikon camera. This settings can be altered using the dial mode at your camera’s top.