Here is a complete mini-guide to league play:
You must have a 60 card deck
You must have at least one basic pokemon
You must have translation sheets for foreign cards (Most people will know what a claydol does, so that type of card probably won't need a sheet)
You must Play by tournament gameplay rule
Usually, tournament penalties are subsided. If you make a mistake, it's usually just curdious to offer your opponent a penalty card, but game losses are rarely issued.
You Cant:
Have unsleeved cards with alt. Backgrounds, such as world championship cards, and cards with a different back.
You Can:
Use previously banned cards (slowking Neo, Sneasel Neo).
I'm sure I'm missing something, but that's really all you'll need to know when playing in league.