Who are you going to see will depend alot on it and how old are you.
but..
1. Concerts usually finish 10:30-11
2. Have a few drinks or whatever else you fancy and have fun, you won't even realise you've been standing for that many hours. Trainers that are comfortable.
3. you will sweat, make sure you have WATER. buy it from the bar, should be £2-3 (rip off I know but you need it). Bring a jacket but leave it in the cloakroom, wear a bum bag/pouch that you can fit a bottle of water into, it'll help a ton. Backpack if you must. Don't wear too much clothes. As little as possible if anything. (top and shorts/jeans whatever) wrap your hoodie around you if you have one.
4- Depends what music, if anything stay out of mosh pits, don't get involved if you don't want to. Stay to the sides of the stage more if anything.
5. Pouch/bum bag that can fit a bottle of water. Backpack if you must
6. none, you won't be hungry, eat something before and on the way back.