Football games are usually at night. My camera is a canon rebel x. You need something like a 300mm lens.
Don't get a zoom, the zoom will have too small a max apeture. (like 5.6) you want to get as much light as possiable.
Remeber, you will have to use a shutter speed of at least 150 (half the amount of the zoom) to keep from getting wiggles in the shot.
Or, if you cant get a zoom, you could try a teleconverter. It is basically a magnifying glass for you lens. It will turn a 50mm into a 100mm (they come in different powers.) They aren't as good but they are cheaper. I agree. A 300 or 500. You will also might need a tripod because of the slow shutter speed. Hopeful it is lit well so you can get those stop action shots (in which case you might not need the tripod after all). | 