To get going, just ask some friends and family to join you for a few weekly walks. If you enjoy these walks, and you make sure other walkers enjoy them too, then your're ready to start a group.
Then you can decide what kind of walking group you want: Is it mainly a social activity? Is it to reduce stress? Perhaps you want to help people who are new to walking? Maybe you want to use walking as a way to lose weight? Or maybe your walking group will explore your area? Decide how you want to feel about the walking group, and how you want others to feel about it.