I've been running a game like this at a local video game store for about 2 years. Consistency is key - you will have weeks where there may only be 1 or 2 people, and weeks where there are 12. Just keep showing up every week (two weeks max, any longer and you'll lose momentum), and when people start eyeballing you from a close/not too close distance find a spot to take a quick break (PC's talk amongst themselves) and say hello and hand out a flyer that answers 'common questions' (I.E., we are playing D&D 5th edition, this game is targeted at new players, please see the FB page or email firstname.lastname@example.org for more information). Cool artwork helps. Keeps questions down, be prepared to tell strangers that their character story is very cool, but you are trying to help new gamers, and most people will support you beyond your wildest dreams.
As a side note, I get about a 20% hit rate on interest to people actually showing up. I have an amazing group, mostly middle and high schoolers (my kids who play are 11, 13, and 17 so fit right in), with occasional adults joining for a few months at a time. Don't be afraid to have a few table rules (1. No swearing (public space) 2. We are all here to have fun, so be cool 3. If something bothers you, bring it up during the game or let me know after. My telepathic powers are terrible, I need you to use words. - something like that helps). Then model what you want to happen - the first time a player wants to chop a head off and mount it on a wall, they're having fun. When it's another PC's head, IF it is not fun for that PC you refer to rule 2 and remind everyone we are all here to have fun so let's do something different.
Hope that helps!