Having gone through just such a feeling only a few months ago, I can easily relate to what you are going through.
My best advice would be to relax, be yourself (no matter how painful that is) and talk as much as you listen. Masons love to talk, but they also love to learn about each other. Be willing to tell funny (not too embarassing) anecdotes about yourself when appropriate.

And remember that you asked to be there, so make the most of the evening. Feel free to ask questions, but don't feel bad if the answer you get is something like, "that will come in time," or "you'll have to learn that for yourself," or "be patient." Everything is eventually revealed as you are ready to receive it and no sooner.

Just be happy that you have found a group of men that may just be friends and brothers you've just never met....yet.

Any report on how you went ?