We started a D&D game at the new comic book store in my neighborhood, and I thought I'd share how it went, in case that is a route you choose to do.
Two of us had separately approached the owner of the store (who has a large downstairs event space with tables and a mini-fridge) about organizing RPG events there. The other of us went ahead and set up a date and time for "introduction to D&D." The owner sent an e-mail out to the store's mailing list, which is how I heard about it. I don't think there was any other publicity. A separate e-mail went out the day of with a different time.
I showed up with 30 pounds of D&D in my backpack. Just the organizer was there. We sat for 45 minutes discussing D&D, our respective gaming groups, the 5th Edition vs 3rd Edition vs 4th Edition, etc. Two people came down who were curious to see the game being played. They realized that we hadn't started playing and, despite our warm and welcoming pleas of "STAY WITH US!!!" they claimed to have dinner plans and left, but did ask about which kind of dice to buy and promised to come "next time." We talked about D&D for another 45 minutes and began to lose hope.
The owner of the store suggested, "if you move the table in front of the register, I'll play." (He had never played D&D before, but took pity on us.) The organizer had the starter set with him, but since we both had played through Klarg's cave countless times, I offered to run this little dungeon I'd scribbled down an hour before hand (that was not nearly as well fleshed out as I intended it to be.) I passed out some pregens and we got to playing.
Right as the characters were scouting out the entrance room to the dungeon, a teenager walked in, saw us playing D&D at the register, said, "are you guys playing D&D? I've never played?" A character sheet was warmly and welcomingly shoved in his face and he was invitingly told to sit his ass down and grab a d20. We started dungeon crawling. People would come into the store to buy comic books, the owner would jump up and help them out, and we kept playing. People were mostly amusedly curious about D&D. Somebody came in to buy some Yugioh cards (young adult, probably past school age) and asked if he could watch us play. We offered him a character sheet, but we were close to closing time and he decided it didn't make sense to roll in with only 30 minutes left.
The three characters got a little split up and got into it with two furry, long-clawed humanoids (slightly modified orcs). The comic book store owner ran away. The other two characters were knocked unconscious and dragged off to an oubliette. The end. The observer wants to come next time. The school aged kid asked if we were playing again this Sunday because it was awesome. (Sadly, we are not.) Hopefully next month we'll be a little more organized.
tldr: As long as you have the DM, a public space, the starter set or a half-baked napkin dungeon and two people willing to humor you and put up with D&D for an afternoon or an evening, you can start to create interest. It might take a few tries to get enough people to siphon off a proper "gaming group," but we had a half dozen people express interest in playing D&D with us at some point down the road, and I know interest will grow as we keep holding the event. We plan to do this monthly.