Well grasshopper you have to decide first what is more important to your shot... long depth of field/ short depth of field or is stopping action more important. Only you will know that. If depth of field is primo in your mind, set it first. Then match the shutter speed to it. Keeping in mind you might have blur if it is too low. You want to stop action, set the shutter speed first... keeping in mind you might have a short depth of field and focus problems. So you need big depth of field and stop action. Then you might want to raise the iso so that you can increase both fstop and shutter speed. High iso may also come at a price but hey it's all a trade off. Also iso is usually raised in low light sitations to avoid the need for a tripod to hold a camera at slow speed. You should be an expert now... So go shoot your camera on manual mode. now I leave you in the hands of the experts.