what would be a good topic about football that i could teach?

This assumes that you are talking about American Football:

If you want to do something with substantial science content, you could talk about the physics of the game. There is a professor at the University of Virginia that annually gives lectures on the subject. If you follow the link below you can get his powerpoint and a transcript, I believe. Could be a good starting place.

Other ideas that would seem interesting to me:

1. The economics of college football. American colleges spend hundreds of millions of dollars, perhaps over a billion, annually on football. Likewise, they take in huge amounts of money from ticket sales and endorsements. But is all of this a good thing? Harvard, Yale, Princeton, and Chicago don't have big time football, but they seem to be doing pretty well for themselves.

2. The "high school football star" and his place in literature/pop culture. This form is nearly ubiquitous in modern America. Why? Why the football star? Why the glamor attached to the subject? At the same time, the "high school football star" is often the supreme manifestation of unfulfilled hopes and dreams, like the uncle in Napoleon Dynamite. Is this simply the bias of authors who were not themselves athletes showing through, or does it say something much deeper about America?

3. The development of popular criticism of football. In the early twentieth century it was terrible dangerous, so new rules were instituted. Its how the forward pass developed. Lately, many people have argued that college/professional football players are little more than latter-day slaves. How did these criticisms develop? What do they mean?

4. Football and politics. Think about all of the former football players and coaches who have entered politics. Tom Osborne and Heath Shuler come to mind quickly, but there are many more. Why on earth do Americans elect these people to high political office? Perhaps it isn't simply America; Briton have certainly sent footballers (ie, soccer players) to Parliament.

What about the history of the World Cup which started in 1930? You can then learn about the countries of the world at the same time.

Here's the FIFA World Cup homepage:

http://fifaworldcup.yahoo.com/06/en/

Wikipedia has some great stuff too:

http://en.wikipedia.org/wiki/FIFA_World_Cup

Perhaps when you have more time you could make a world cup replica!

What type of class is it (math, history, etc..) would help. As a History teacher I would teach my classroom about the history of the game and the social, political, and economic issues that surround the sport.

Good Luck!!!

Predicting outcomes on the law of averages.

Statistics & graphing to find out which team has won the most/least games.

Evolution of the various codes of football

Rules of the game

Signals & codes

a specific technique in football, or the specifications of the ball, or something like that.

Steroid use