slenzo: great question
GBC Chairman : great answer
Finding a league if you are new to either OOTP or OOTP-style multiplayer gaming can be a daunting task. My personal experiences have happened to many people: The fact you don't know very much can doom you to errors so major that it can be disheartening to continue. You can get huckstered into a trade so bad that not only did you weaken your team a lot, but the other GM's are upset at you for giving so-and-so all those great players for nothing

. I bet many people gave up on online leagues after a particularly rough start like this.
Finding a patient and understanding Commish is a great help here, and don't be afraid to ask question either here or in your league forum or via email to the commish. Some leagues have been successful with rules to restricty trading during GM's first season. The reasoning is to prevent collusion style cheating where GM#1 has his friend take a team and trade all the best players back to GM#1. It also will help avert duplicate GM collusion where a GM runs 2 teams under 2 different internet nicks

. A 3rd reason for the first season trade restriction is to prevent the shark GM's in the league from taking advantage of newcommer GM's - after a season of play the GM's will understand the game and the league much better and be able to make trades on a more level playing field.
All that said there are a lot of great leagues out there with patient Commishes and owners who accept newcommers all the time. Often there is a wait for these leagues but if you have good communication skills an introductory email to the commish will often land you a GM spot quickly. Maybe the MBBA will be the perfect way to learn the game and/or online play.
Once you have played in a great online league it is very difficult to play the game solo

. Good luck.