You're going to be nervous. Insanely nervous. Shaking like a leaf in a thunderstorm nervous. Unless you're one of those people that actually loves public speaking--in that case every author in the world probably hates you. Kidding lol.
There will probably be a display of your books in the store. Mine was right by the entrance, staring at you when you walked in...how cool is that?! So take a minute, pause, breathe, and get a picture of that display. Book stores don't often give away that much shelf space for a book so it's a moment to be cherished. I didn't breathe all night... but my mother-in-law got a picture for me :)
Next you're going to find the area you'll be set up in. There should be an event coordinator of some kind to help you out once there. They'll let you know how things will proceed and make sure you have everything you need. If you forgot a pen or need a glass of water, this is the person you'll talk to.
Also, you're going to want to make sure you set yourself up for this event to be as successful as possible. That means inviting friends and family, as many as you can. Nothing looks less inviting and less interesting than a bunch of empty chairs. You want people to wonder what the big deal is when they walk by. On the same token you do want empty chairs to be available for people that do walk by. It's not very inviting to want to stop and listen but not have a place to sit.
On that note, try and get people to sit near the front. Leave the empty chairs in the back. That way if someone walks by and wants to stop and listen, they don't have to stand or make their way to the front, they can just inconspicuously grab a chair in the back.
When you get up to do your reading, take deep breaths, smile, and enjoy. Have a script written or cue-cards for what you're going to say between excerpts or before and after the reading. You don't have to use them, but they're there if you freak out and your mind goes blank.
The formal section of my book event went on for about half an hour. I think this is pretty normal, so time your reading accordingly. I had about 5 minutes of intro between the book store event coordinator and the person who gave me an intro. Then there was my reading, which I timed to last about 15 minutes. If you want to talk about your excerpts, do so. People like to hear behind the scenes stuff. Or, you can do a question and answer period after the reading.
Here's where I made a mistake. When opening your question and answer period, don't freak out if no one asks questions right away. It takes time to think of them and it usually takes a couple people asking questions to get the ball rolling. I had a couple questions asked, then it got silent, I got nervous, and I closed the formal part of the evening and we moved on to the signing. If I had to do that again I'd wait a few more minutes, let people come up with a question, maybe have a few interesting facts pre-written and ready to share to keep the silence from being awkward.
During the book signing part converse with the people that come to your table. They took time out of their lives to see you, which is pretty cool. I had one lady come up who just happened to be in the store, heard the announcement over the PA that I was doing a reading and recognized my name... she is my husband's great aunt! She had no idea I was an author, I've never met her before that evening, but she just thought this was the neatest thing and bought a book. If I hadn't talked to her, I might not have learned that.
Remember to ask who you should make it out to and ask for spelling. These days a name can be spelled a million different ways...you wouldn't want to misspell it when signing a book to them.
At the end of the night thank the book store employees that helped you out, and bask in the fact that you just completed your book event. You're a rock star :)