Soccer league ER diagram (implementing a many to many relationship)
Do a Google of "open source soccer league software" and you will find a plethora of offerings already on the "market". My advice to you is to download a few of these, play with them (use last year's results). 

See which ones come closest to what you want to do. Then take a look at their database schemas (and code if it's written in something you can read) and adopt and adapt the bits that suit you and leave the rest.

Unless you're planning to do this as a hobby project (and even then), but if you want to get up and running quickly, this is probably a good strategy.