Oh man...shoot a LOT EARLY, before the sun goes down!!! Every minute the game goes on, the light is going to grow dimmer and dimmer and dimmer. Keep the lens at the shorter end of the zoom range, and set the ISO HIGH!!!!! I would suggest using AUTO ISO, and setting the Aperture to f/4, and shooting at the SHORT END of the zoom range, so the aperture will stay at f/4....as you zoom the lens to longer focal lengths, the aperture will of course, start to go higher in number...f/4.5, f/4.5, f/5, f/5.6,etc, meaning less and less LIGHT comes in...
I would say, as Big Mike mentioned, shooting in MANUAL mode on the mode dial, lens aperture set to f/4, with AUTO ISO enabled, and a shutter speed of 1/250 or so, and the AUTO ISO set to as high a maximum as it will go!!! Once the sun goes down....ehhhh...at "most" high school fields, you're going to have to be at ISO 6,400, with the lens at the widest aperture range, to get a decent shutter speed. I am basing my comments on estimating your skill set and knowledge. I can tell you this: an f/4~5.6 variable maximum aperture zoom lens is NOT WHAT I WOULD PICK to shoot night high school football. The best light will be the first half of the first quarter of the game, so shoot the DAYLIGHTS out of that part of the game.
Now, you COULD also try using some flash as fill-in lighting, with the flash set to Minus 2.5 stops....a tiny bit of flash as FILL-in lighting is an old, well-known technique that "some" shooters have used for decades on night HS football. When done well, it looks nice. You are not trying to shoot the exposures based entirely on flash--no, the exact opposite...you are using an elevated ISO setting, like 1600, at around 1/180 or 1/200 shutter speed, and FLASH, set with the flash power deliberately dialed wayyyyyyy down, to MINUS 2.5 or Minus 2.7 f/stops (also known as Minus 2.7 EV or Exposure Value(s) ).
The thing is this: you have a d-slr that develops the pictures right there, on the spot. As a beginner, you'll need to use the tools you have, not the great gear some of "us" have. And, you have little experience to draw upon. The best thing you could do is to ASK FOR HELP from another shooter who is on the sidelines, and who knows what he/she is actually doing. You might need to LOOK carefully at your captures during the game to see if yuo are getting usable shots, or garbage.