First you typically want the lowest ISO for your application. Second you need the fastest shutter speed to be able to avoid camera shake and freeze you subject. Sports you want a fast shutter. 1/1000 or better. Someone sitting 1/60 will do. Rule of thumb for avoiding camera shake is shutter should be be one stop faster than the focal length. 50mm = 1/60. 300mm = 1/320. YOu can lower that a bit if your lens has IS. I usually always go much higher. Better for me.
Next is DOF which you control via the aperture. Shallow or deep. Shallow means a persons face is in focus and everything is out of focus. Deep means everything in the image is in focus. Shallow = 1.4. Deep = 16. Here is where you are creative. Not to say you can't be creative with shutter speeds. You may want motion blur.
DOF calculator. Distance to subject is very important when you are shooting shallow. http://www.dofmaster.com/dofjs.html
I will always sacrifice ISO and aperture if I want to freeze my subject. I can fix noise from high ISO and I can live with a blurred background if my subject is in focus if I need to open up the aperture to maintain shutter a fast speed. You can not fix a blurry, OOF image.
Everything above is a slave to exposure. You have to keep the camera's light meter in the middle - for starters. Later you need to learn how your camera meter works. How to expose for back lit subjects, how to expose for pure black or pure white subjects. Those are extremes but you need to know that. A 50% black and 50% white subject will expose properly when the meter is in the middle.
Play with this for a while and you will start to get the hang of it. http://camerasim.com/camera-simulator/