"Simple" is a very relative thing. 😉 Without knowing your particular skill level in programming in general, PHP in specific, and database design and SQL; it's hard to know exactly where to point you first.
The main design challenge for such a site may well be coming up with an optimal database design. It may be that you'll need to break the data up into more than just those 3 tables (I've not given it that much thought, so maybe not). If you are not real strong in the database area, you might want to do some reading on "database normalization". A couple places to start:
http://www.geekgirls.com/menu_databases.htm
http://dev.mysql.com/tech-resources/articles/intro-to-normalization.html
Alternatively, instead of building this from the ground up, you could search some of the usual suspects (hotscripts.com, sourceforge.net, etc.) or Google for an open-source sports league/team web site package that gives you the functionality you desire. Of course it's caveat emptor, but I have to think there must be some good packages out there, as I'm sure you're far from the first person who has wanted to create such a site.
