Since i see reddit was mentioned , which is an outside website, i would also add DaddyLeagues, as it still has alot of leagues hosting their sites there while waiting on this app to drop.A forum where people can post openings, a "find a league" page also where commishes can post openings as well, Plus just a league search , where you can go search leagues by console choice ,and look at their pages, and get in contact with someone. So there are lots of options for people as the ones mentioned above in thread as well as here to find a good Sim league , where the guys are not there to play lobby ball.
http://daddyleagues.com/forum
http://daddyleagues.com/top20/madden
http://daddyleagues.com/elite_leagues
Main thing, as mentioned in the article, is find a league with a good commish and admins that are not just trying to make themselves the top players and are fair and enforce their rules for everyone in league equally.Communication by all owners in an online league is also a key component to keep one running well because everyone has busy lives.A league thats run fairly by commish and admins, will be one that will last and provide you alot of excitement for a long period . Once you find one like that, you wont want to leave.
As a commish , i also verify what Chris said about commentment , do not join more than you can reliably handle.Find out how often they advance from week to week to start off with.And from there figure out how much you have time for . No point in jumping in one, if you dont have time to give it the amount of time needed to be active with the rest of its owners.This is a common problem commish's deal with alot with new owners, over stretching themselves. So be sure to ask commish that first off , after reading over their rules to figure out if its for you or not.
If you havent tried it yet, once you find a good league, you will be glad you did. Good luck!