There are tons of free fantasy football leagues to join, yahoo.com, espn.com and nfl.com are to just name a few. I used to enjoy the free leagues when I first started playing fantasy football for obvious reasons, but there was one major problem. Almost every single free league people don't actually follow their team throughout the draft. If there is an online draft you'd be lucky if half the people show up. Playing in these leagues isn't so much fun if no one is taking it serious. I end up playing against people who have 3 starters on bye weeks and I win easily.
This year for the first time I joined a pay to enter league. This particular one is with friends and family, and was $140 to enter. All the money that is collected is spread out to the 1st and 2nd place finishers along with weekly winners, so you do have a chance to win a lot of money. The money isn't so much the important part though, its more about the dedication everyone puts in. There is no such thing as team owners playing people on a bye week. People actually use the waiver wire and offer trades to each other.
If $140 is too much there are plenty of online sites where you can pay $20-40. With this small fee there is a much better chance that everyone in your league will take it seriously. Usually there are great prize structures associated with winning as well. So if you've played fantasy football for a couple years I still suggest trying the pay leagues, I think you'll learn alot more and have much more fun.