What should i wear to my cousins wedding?

I'm a 13 year old girl around 5'0 - 5'5 i have light blue/hazel color eyes blonde hair down to my armpits. Its parted straight down the middle and isn't straight but its not curly either, i guess it's kind of wayvey. Anyway i was thinking about getting a dress from rishi because they have some really nice ones. But i have no idea what type of dress to get my nana is christan so it has to be acceptable. I have ABSOLUTLY no idea what to do with my hair :( or for shoes wise! i have really long legs and i'm a little bit tanned oh and i have freckles :)