How do I plan this preteen party for an 11 year old girl?

It's clever to have a swimming party because the children will, for the most part, entertain themselves. Adolescent girls love to just gossip with each other and come up with their own makeshift games! However, you can help by making simple but cute invitations for her to give to her friends (be sure to ask for an RSVP, so you know how many kids you'll be handling). You could supply the food she wants, but be sure to add something at least moderately healthy so that they don't run out of energy or feel sick. I don't think decorations will be necessary, but you could put up a birthday banner if you want (at this age, kids aren't so keen on having decorations, they might find it corny). You could ask her what games she wants to play, and then buy small prizes (like tattoos, gel pens, etc.) Maybe you could buy them a selection of movies in case they get bored before they sleep. Don't worry too much though, at this age they really just entertain themselves. Good luck, hope your daughter enjoys herself.