RE: Ask a Simple Play! Pokémon Question
If you already have a Player ID, you will need to bring it with you. If you do not have one, a Player ID will be issued to you when you go. You will need to keep this card with you when you attend events. You will for sure want to bring your deck, damage counters, and a randomizer. Many players will also brings cards for trade with them.
Some leagues do support the Pokemon Video Games. You will have to ask your league if they do if that is something that interests you. My league I do support it, but not all leagues do.
The most important thing to do at league is have fun. You will probably learn a lot if you are new to playing the game.