With absolutely no budget you're at the mercy of the marketplace. You can go corporate hellos (door-to-door selling) if you're in B2B, and home to home if B2C.  The only thing posting on social media costs is time, but without even a modest budget you've got a very steep, uphill battle. 

Find a local restaurant or coffee shop and see if you can get some space at no charge for a small gathering of people that you'll invite to a "Free Seminar" on your topic. The only way to post it online is to use social sites like this one to advertise your event to drum some attendees. 

I posted an article a year or two back where I described a co-marketing opportunity with a local Charity. No time to spell it all out but you might find it here somewhere. Anyway, contact the local charity or charities that might have your avatar and ask them to push the invite to their contact list in exchange you'll give them the donation attendees will be asked for at the door. I'd put that info in your ad or you'll be pulling black hat s***. You don't want to offend. 

That's what I have for you, my friend. Good luck.