Find if your city already has organizations, if not the easiest is to start a 50501 group and push it on social media and make an event on mobilize.us. Pick a busy intersection and it will grow every time you organize the next one.
Don't compete with other organizations, unite and collaborate.
I started with 10 people in my red city and then helped organize a protest of 4k people within 6 months. You'll see how much people want this.
Have thick skin as well. My family and I have been threatened and harassed by the local MAGA idiots who just want to start shit. Good luck.