Cheap/spare PAYG phone, pref one that holds charge or 2 that don't and you don't care if they get nicked.
You don't need anything valuable.
Don't bother washing for the weekend - babywipes and dry shampoo are wonderful inventions.
Loo roll, binbags, spare blanket in case you end up sitting round a fire and it's freezing!
Def padlock the tent and don't accept anything from a stranger - plead food intolerances or asthma if necessary. Chances are you'll never see them again and even if you that's cos they're nice enough to understand!
Have a meeting point arranged with your friends ahead of time in case you get separated and a time limit from when you notice they're not with you AND you notice they're not at the meeting place. Sods law dictates if you don't then you'll get separated and mooned phone will have battery. Try to move in a pack...
Small first aid kit makes you very popular antibacterial alcohol hand gel is practically the best invention ever. As are cereal bars.
Pack layers for weather extremes. Shorts can go under baggy jeans/combats, vest top then t-shirt then hoody plus lightweight mac. Binbags are an accenpatble substitute if you lose your mac! Whatever you do don't wear sandals or flip-flops!!! Take wellies and thick socks (various pairs). If it rains wear shorts and wellies - skin is easier to dry than clothes, especially at a festival
Don't tell your parents what you did when you get back - instead be non-committal, have an hour long hot shower, leave the washing for your mum, sort the tent out or it'll get mouldy and go moon over that cute student with dreads and a guitar