The fear of the unknown is probably hard to shake because after all we're dealing with people here - it's completely unpredictable what they will do. But I can tell you about the meeting format and what's expected of you.
Meetings generally fall into three parts: introduction, general sharing and ending.
During the introduction parts of the ACOA literature is read. Which parts varies a lot between different meetings but it usually includes the Problem, the Solution and the 12 steps. The fourth tradition gives each group the right to make decisions about their meeting format.
During the introduction you'll be told things like if there is coffee, if smoking is allowed and you will be asked to introduce yourself so that the other people in the meeting can welcome you. Give your first name only and allow the group to say hi. Some meetings also have a check-in in which everyone gets the chance to say how they feel then and there. If you don't want to participate, just say your name and that you just want to listen, then pass the word on to the next person.
At the end of the introduction the secretary (who leads the meeting) will say "I now open the meeting for general sharing" or something like that. If you sit back and listen you'll learn a lot. If you want to share, different meetings have different rules for how that is done. In some meetings when the previous sharer has thanked the meeting for listening, you just blurt out "Hi! I'm Apel, I grew up in an alcoholic home" to get the word, in others there's a waiting list and you raise your hand to get the attention of the person who has the list and be put down for sharing. There are a lot of other variations too. It should be explained during the introduction. If it isn't, just watch what other people do and follow their example.
You'll soon figure out that no one else is allowed to talk during other people's shares, not even for appreciating comments. Feedback is also not allowed. Your share is supposed to be about yourself and your own experiences, not about the other people in the meeting or about alcoholics in your life except how their behaviour affects you. You can however share about things that other people have brought up. Saying something like "What's been shared previously reminded me of a situation where I..." is fine.
5-10 minutes before end of the meeting the secretary will say "We've come to the end of sharing time at this meeting" and start to wind down. It's during the ending that the pot is passed so that people can donate towards the rent, photocopying and other meeting expenses. Give only as you can afford. In many meetings newcomers aren't allowed to contribute, they are encouraged to put money towards literature instead. You'll have a chance to buy inexpensive leaflets about the steps, how to share etc after the meeting. Some meetings also sell the ACA Big Book.
Most meetings end formally with a prayer. Usually everyone stands up and holds hands and says the prayer and ends with "It works if you work it and work it, you're worth it!"
Some meetings will have volunteers who talk to you after the meeting, showing you the literature, giving you a list of meetings in the area etc. In others you can ask someone who seems to have been there before. Getting a meeting list is particularly important.
I hope this has given you a better idea of what to expect and what will be expected of you. FYI most of the above applies to all 12-step meetings, regardless of the fellowship. The ACA World Service Organization maintains a list of ACOA meetings worldwide.