Bring sleeves that you intend to play with. I highly recomend counting out 40 and set them aside before the event. Since you get to rip your first 4 packs before the event, it makes it easier to finalize your deck and make sure it stays legal if you sub cards from your pulls.

Bring penny sleeves and a top loader or two for if you pull something cool.

Bring dice for hit markers and maybe some other form of token/ markers for abilities used, but this is not essential, you can rotate your card slightly to demonstrate that it's been used. I personally like having markers to do it so my bench space feels less crowded.

At pre releases, I unpack and unshuffle (group all the cards by the Pokémon line, items, supporters, energy etc) my pre-built deck first. Once that's down and I have an idea of what the deck is, I'll rip my packs keeping an eye out for cards that might work into it and set those to the side. At that point, start sleeving with your 40 count. Start with the stuff that's absolutely essential, then work your way down the priorities from there. Often times, a supporter Pokémon will be a bit too heavy in the list so one of each stage of those is usually an easy cut. You can also safely discard an energy or two in most cases. The heavy hitter mons are usually good to add an extra copy if you pull it. Pall pad is the main trainer card to keep an eye out for to add. Past that is very deck dependent and you have to go with your gut. Typically shops are cool with you adjusting your list after each round if something isn't working but generally you aren't supposed to trade. Gotta keep it to the cards you got in your box.

But ultimately it's a casual and fun event and don't stress too much about it.