Oh, if you are in "S" mode, you won't be able to change the aperture, and if you're in "A" mode, you won't be able to change the shutter speed. "S" means you set the shutter speed and the camera decides the aperture for you. "A" means you set the aperture/f-stop number and shutter speed is automatic.
If you want to be able to change both shutter speed and aperture, go into manual mode. You'll be able to fiddle with all points of the exposure triangle in "M" mode. For action, if I weren't shooting in manual mode, I would pick "S" because I can set the shutter speed I need to stop action. If I were in Aperture Mode, I could pick a low f-stop number but the camera might not pick a shutter speed that was fast enough for me.
If you just want to just try shooting in "S" or "A" mode, remember you can always change ISO to brighten up the image. If you find yourself having to shoot at shutter speed 1/1000 s the image will turn out very dark unless you bump up ISO to compensate (or fix in post processing). I usually always adjust ISO a little bit, but I don't worry too much if the picture turns out underexposed because I edit them anyway
I posted these pictures before as examples, and I think I was at f/2.8 to f/3.2, shutter speed 1/400 to 1/1000, and ISO 1250 all the way to ISO 3200: agility picture
one,
two, and
three.
