Poll coworkers for their interest before you start planning the league. Talk to everyone so you understand the overall interest in forming a softball league.
Bring up the idea with management, emphasizing that a recreational softball league is a good way to foster teamwork and build morale in the company. Let them know the degree of interest in the company. You may be able to get some funding for the league.
Decide how to structure your softball league. If you work at a large company, different departments can function as individual softball teams so you can play against each other. If your company is small, you must find other small companies to play against.
Recruit a few coworkers to work with you on the softball committee. A single person can't start a softball league because there's too much work to do and too many decisions to make.
Create the rules with the other softball committee members. Decide how many people should be on each team, and whether to play slow-pitch or fast-pitch softball. Also consider how often the teams should play and whether they should compete internally or against other companies. Also decide whether prizes or trophies should be involved.
Investigate costs and equipment. Find out if anybody in the company has softball equipment to donate. If not, you must purchase new equipment. An inexpensive T-shirt with a company-inspired logo makes a good uniform that each participant can purchase himself. See if you need to pay to play softball at a local field or if you just need to reserve the space. Charge participants in the softball league a set fee to cover any costs the league might incur.
Hold an organizational meeting after work, inviting anyone who's interested in softball. At the meeting, explain the rules and fees, and let people know how often they'll be playing. Sign up people for teams at the meeting.
Arrange games between your internal teams or contact other small companies with softball leagues and set up games.