You probably will want your organization to meet a few basic tests before your first meeting, such as knowing who your other key organizers are and how you want to involve people. Keep in mind that your larger goal is to grow your organization or movement into something that has real power. Get together with the people you consider your leadership group and work on a goal or set of goals for the meeting. Try to gain consensus there and begin to develop roles for each person in that group. This is a smaller version of what will happen in the full meeting.
Some meetings require a very open structure to enable members of the group to be heard.
These meetings can be chaotic without some limits being built in, so choose a strong moderator who can field and organize multiple ideas from a large group. Make sure you have a system, preferably highly visible, for recording ideas, so people know they are being heard and can easily respond to others' ideas. A featured speaker, a video, or a slide show can be a great tool for getting a message across, but your audience wants a chance to comment, too. Don't let the educational message go on too long without questions and answers. Some organizers find that they need only to put signup sheets on tables to get the work done, but most people need to tell or retell their story first.