Suggest going to M mode on camera. Set ISO to 6400, aperture wide open, shutter speed at 1/1000, continuous auto focus, shooting hi-speed shutter (if available) and adjust from there. If you can't get fairly sharp shots after adjustments, you will need a faster lens. Just the way sports is. Shooting sports in low light can be very expensive. You are having a great first effort.
Shoot early because the light degrades by the minute. Also, it helps to stand beyond the end zone the offense is moving toward and "let the game come to you." Just being that much closer keeps you from having to zoom so much. Or spend $6000 on a a pro level camera and a 70-200 lens. Don't.
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