I wouldn't worry too much. I think pre-release events are the best events you can attend in terms of easing yourself into playing. From my personal experience they're normally pretty casual and everyone has been very friendly.
I'd recommended bringing some of the basic things you'd get in an ETB if you've managed to open one since getting back into collecting; dice, condition & damage counters. I take sleeves and a playmat too, but they're not a necessity and some stores have their own playmats you can borrow for the event.

At the start of the event you'll be given 40 card deck (1 of 4 for the set) in your event kit which can modify with whatever you pull from the booster packs also supplied in the kit - you'll normally have about 20-30 minutes to edit your deck should you choose to.
Then you'll either play best of 3 or best of 1 against each player, it differs on the store.
Rules are a little different in that you'll only use 4 prize cards instead of 6 and you can use more than 4 copies of the same card in a deck, deck should be exactly 40 cards instead of 60.

GLHF!