What should i wear to my friend's birthday party (I'm 16)?

Most of her family will be there and it's only going to be me, 2 other friends and my friend who's birthday it is. I've never actually gone to a party except from when i was younger (like 10) cause seen as i have Social anxiety i don't tend to speak to many people at school so i don't get invited anywhere and now that i have been invited i am really nervous and i don't want to dress down but if i do then i'll probly wish i hadn't but if i don't then i'll feel too dressed up. So What should i wear? I have dark sparkly denim jeans (sound worse than they look) and a sparkly top the same colour and a kinda loose black shirt to go on top. By the way there's a barbecue and the party starts aroun 5ish. Thanx for any help:) xXx