I've camped at quite a few festivals. My top tip would be to not drink so much on the first night that you feel awful the next day. An upset tummy and portaloos are no fun!
Otherwise, prepare yourself to have to walk a long way from the car to where you pitch the tent and pack with this in mind. A trolley is a big help, some festivals have them for hire.
I always take a camping stove so I can make a cup of tea in the morning. You can also do cuppa soup, pot noodle, etc for lunch. Festival food is expensive so I would try and bring stuff for breakfast and lunch and just buy dinner. Being a water bottle that you can fill up so you won't have to pay out for soft drinks. I normally bring alcohol for the first night but after that I buy it on site as I don't like drinking warm drinks. It's pricey so take plenty of cash or you're going to be drinking!
A bum bag comes or bag that crosses your body is a good idea so you can keep your hands free when dancing, etc. Carry packets of tissues, wipes and sanitiser as the loos will be gross.
Take a lamp for the tent and a clip so you can hook it to the top of the tent. It's horrible having to forage around in the dark for things. Also you'll need a torch.
Bin bags for rubbish. Keep a set of dry clothes in the car. If it's wet and muddy, you'll really appreciate having something dry and clean to travel home in.
Mini first aid kit - paracetamol, cream for bites and stings, plasters
Power pack so you can charge your phone. Warm clothes for the night time and blanket. Wellies. Flip flops if you're planning to use the on site showers.