Basically, you go through some registration stuff to get your own team; you then choose a name for the team and for your arena and a location. (They prefer you to choose the region you're actually living in, but you can be anywhere in the world.) The site gives you a small supporters' club, a team with a fairly poor coach and twenty or so horrible to mediocre players and places you in the lowest series in your country's league. There's an economic model in which you pay players and coaches per week, buy and sell them on the transfer market, upgrade your arena, hire assistant managers and scouts, pull players from the youth squad, and receive money from sponsors and game attendance.
It's pretty cool (in a very nerdy sort of way). If you want to start a team or already own one, I'm the Newaygo Lions in USA Series VI.