Football is actually pretty easy to photograph if the stadium is open and the sun is shining. I managed to get two shots in the Pntax Photo Gallery, both shot with a consumer lens, and from afar (FA 100-300 f4,7-5,8). Instead of working with shutter speed, I suggest you set your camera to the highest ISO value you feel confortable with, and use the widest aperture that you you like on your lens. Then pre-focus as much as you can, and if you can keep both eyes open, one in the viewfinder to see what you will capture, and one looking at the field to anticipate better the action.
Practice makes perfect. Here's what you can expect: