My experience is that people often have questions about “controversial” moral questions: embryonic stem cell research, in vitro fertilization, contraception, abortion, euthanasia, etc. And then they usually have questions that are more on the periphery of doctrine: why do we genuflect, what is holy water for, etc. Then there are more difficult doctrinal questions (difficult in the minds of the questioners): women priests, homosexual “marriage”, divorce and annulments, etc.
So, one way to approach this would be to advertise a talk and discussion about a “controversial” topic. That’s the initial draw. Just advertising a series on apologetics may not attract people, often enough, because they don’t know enough to realize what they don’t know. So there has to be a hook. If the presentation is solid and meaty, and the discussion is deep, then word of mouth will bring more people.
One could determine all the topics ahead of time, and advertise them up front, or one could have a question and answer box, or simply take suggestions from participants for the next session’s topic. There are many ways to do it.
I also suggest having a good supply of handouts for each topic–people need something to take home for further study. Also, a warm and welcoming environment, refreshments and coffee, etc., go a long way in bringing people and keeping them. God will use whatever crack there is to get His foot in the door!
One format that I find that works is the following: presentation by a speaker, followed by either questions and answers, or else small group discussion followed by reporting back to the large group what the small groups discussed. Deep and probing questions, both for understanding and for personal application, are essential.
If you can’t get a speaker, then a video series can be good–but this lacks the dynamism of a live speaker. Still, if videos are used, a period for Q&A and small group discussion are definitely needed. Many such series are available. Two that come to mind are Scott Hahn’s “Answers to Common Objections”, or else, on a different tack, Fr. John Corapi’s 48-part series on the Catechism. I also recommend Ignatius Press, Our Sunday Visitor, etc. where you can find more resources. Hope this helps.