My suggestion is to start out slow. Look at the game in the IS and go from there. In the US you have 3 major leagues. The top one is MLS, then you have the North American Soccer League (NASL) and the 3rd tier is USL Pro. You might even find something on Wiki that will break it down for you. Once you understand the US system, it should be easier to understand the systems in other countries.
While I typed this, I found a good link...http://en.wikipedia.org/wiki/United_..._league_system