There are different types of softball leagues to suit every type of player, so it's up to you to decide what kind of league you want to host. Do you want the type of league where everyone wears matching uniforms and the play is highly competitive? Or do you want the kind of league where everyone has a glove on one hand and a beer in the other? Whatever you choose, just be sure that it's made clear to any interested participants. Games can go south fast when half the players are serious and the other half are goofing off.
Do you want your teams to be co-ed or all male or female? And then, do you want to have a fast-pitch or a slow-pitch league? Fast pitch is best if you have teams of skilled players, while slow pitch will allow for less experienced players to get out there and be part of a team.