Never been to a Bat Mitzvah! What do I do!?

I have never been to a Bat Mitzvah, and I was just invited to one. I have no clue what to wear (girl), what present to bring, or what to expect! They are having a smash and splash, where we play tennis and swim. I have no idea what to expect for that either, and I can't play tennis very well, while her whole family are super talented tennis players. Help!