What to pack for a trip to the lake?

My sister invited me to the lake with her, her family, my other sister, and her family. I'm a 15 year old girl and I've never been on an overnight trip to the lake before so I really don't know what to pack. I'm not really sure what the situation is, other than the fact that I'll be staying in a huge house and they have boats/tubing/water skiing/etc. I want to look cute but definitely not dress too nicely. I'm leaving Sunday and coming home Thursday.