When photographing football, you are generally not allowed in the team area at the center of the field. You’ll be able to stand in the areas noted as 1 and 2 in the diagram.
Stay about 5 feet back from the sideline to give you room to run if the play heads your way.
A good lens to use for football is a 70-200 zoom lens. When working at night, you can use flash, but will only be able to get plays that are literally right in front of you. Remember . . . even modern flash units aren’t much good beyond about 30 feet (10 years). You’ll have better results with a “fast” f:2.8 or f:4 lens. Set your ISO to about 5000 and you will usually be able to get shutter speeds around 1/500 with the lens wide open (largest f-stop/lowest f-stop number).
Regardless of the lens you are using, don’t try to capture action past midfield. Not only do you need a super telephoto to fill the frame beyond midfield, but you will usually have too many players running in front of you to get a good shot.
Set your camera on the fastest “motor-drive” setting so the camera captures a series of pictures quickly when you hold down the shutter release. Set your focus on continuous autofocus (for Canon, that’s AI servo mode; for Nikon, that’s AF-C).
In continuous autofocus mode, your camera will often grab focus on the background. Set your focus point to a central point in the viewfinder, since your will generally be centered on the primary action. This will give you the best chance for proper focus. Be prepared to discard many images due to poor focus. But also be prepared for some amazing shots as autofocus rises to the challenge!
You will generally want to get about 10-20 yards in front or behind the line of scrimmage, since the action begins behind the scrimmage line and quickly moves up the field.
For offensive action:
Stay behind the line of scrimmage to capture the quarterback as he hands off or throws a pass. This is also a good place to capture the punter.
Get up-field to grab action shots of the ball carrier charging through the line, or the pass receiver. Prepare for action to move quickly across the field!
Stand behind the end zone (position 3 in the diagram) to get neat shots of your players charging over the goal line. This is great for the PAT, especially if your team prefers to stay on the ground.
For defensive action:
Depending on the strength of your team against the opposing squad, choose a position that will have your players moving towards the camera. If the opposing team is dominating, stay behind your players to capture them as they bring down the ball carrier. If your team controls the field, stay behind the opposing team to capture your players as they sack the QB!
Other action shots:
Look for neat closeups of players on the sidelines. And try to capture the coaches, both watching the action and talking to players.
Take a few minutes to photograph the cheerleaders, and consider getting some shots of the trainer working with an injured player.
These are important to telling the story of the game.