English Premier League
A lot of the best clubs are probably sold out. But, look towards the middle or bottom of the league (especially from last season, try this website http://www.premierleague.com/page/Statistics/0,,12306,00.html and choose the 2009/2010 season). Then you will have to do some research to see which club is near where you are. There are also other professional leagues, such as the Football League (Championship, one, and two leagues). These can be a lot of fun because they are more local in nature.
Your best bet would be to just ask the locals. Tell them you would like to watch a football match, and I'm sure they will be more than happy to guide you in the right direction. I have never been to England, but I have bought tickets to Bundesliga games in Germany right at the stadium. This usually works, although once I was unable to get tickets to a game because it was sold out.