It was described a little bit in
this thread, though you have some additional questions.
As was stated there, you're going to be seated at a table with other players (introduce yourself, schmooze, you'll make friends
). You'll then be given six packs plus a promo card (Bronzong for this set). You will have thirty minutes to build a 40-card deck with the cards you pulled, and you may or may not be able to use the promo, depending on the venue. You will also be supplied with basic energy.
You'll then play in a tournament. The staff will post a list of all the players and who they're paired against before each round begins. Find your name, and go to the table number listed. You'll then play your opponent with your 40-card deck, only placing four prizes. Exactly how long these go on can vary. Rounds are 20 minutes each, with some downtime between. You can reasonably expect to be at the event for about half a day. After the main event ends, you will return the basic energies you borrowed and will receive two additional packs.
Many venues will also hold a booster draft after the main prerelease event. You can find more information about how those work in the link I posted. It's basically an opportunity to get more cards, and there are prizes going out to the winners in the form of packs. I'm actually going to write an article about prerelease and draft procedures, aimed at new players in the coming weeks. I wish you good luck at the event!
One last thing: Be sure to bring your trade binder! There will be lots of people there wanting to trade cards from both the new set and sets that are already out. It's also a good idea to bring a pack of sleeves. I usually don't sleeve my cards at a prerelease unless I pull an EX, since sleeving takes time, but it's good to be preparedd. You should also bring your 3DS with you, and leave it in sleep mode in your pocket. You'll get
a lot of StreetPasses.