Start with one conversation
Every lasting community group starts the same way: one person talks to another and asks what they care about. Not a flyer, not a social media post, but a real conversation.
Pick five neighbors, colleagues or parents at your school and ask each of them a simple question: what's one thing about this place you'd change if you could? Then listen. Don't pitch your own idea. Take notes on what they say and how strongly they say it.
After five conversations you'll usually notice the same problem coming up in different words. That overlap is your starting point. You haven't organized anything yet, but you already know more about your community than most people who try.
Finding the shared problem
A good issue for a new group is specific, winnable and felt by many people. "Fix the city" is too big. "Get a crossing light outside the primary school" is something people can picture, talk about and win.
Write the issue down in one sentence and test it on the people you spoke to. If they say "yes, exactly", you have it.
Building a core team
No organizer can do it alone for long. Look for the three or four people from your conversations who were most energized, and ask them to take on one small task each.
Small tasks are the secret. They let people say yes without fear, and every completed task builds the trust you'll need for bigger ones.