I think you'd want to start by studying social movements that did change the world. Figure out what they did, how they got going, what problems they faced and how they overcame those problems. See if you can fit what you learn about past successes into your present context.

I also think you need to balance the pros and cons of online activism. The pros are, obviously, that you can spread a message cheaply and build communities. A major con is that it's easy for people to just do the online part. If people have some impulse to support your movement, and they can entirely satisfy that impulse by participating in a subreddit, you may find that all you accomplish is building a subreddit.

I have no advice or idea about how to balance the pros and cons, it just seems to me that there are pros and cons. A group that just complains about something is easier to create than a group that solves a problem. Maybe the complaining group is necessary to raise awareness - but maybe it is also a distraction.

Finally, I would think you would want to experiment and iterate. Both with your approach to organizing and with your actual solution. There is a benefit in starting small - easier to make changes and less consequences if you're wrong.