Exercise A LOT
Eat proper food NOT frozen schiz.
Do not drink diet coke, it's TERRIBLE stuff. Water Water Water.
Eat lots of fresh vegetables and protein, fish, chicken. Eat a good healthy breakfast. Porridge, poached egg on good quality wholemeal bread with tomatoes, mushrooms, spinach etc
No junk - sweets, crisps, fizzy pop, cakes all that crap has to be put on hold.
Think of it as an exercise in being good to yourself rather than depriving yourself. I would also personally not get too hung up on calorie counting unless you are no good at portion size. Remember unlimited veg but only palm sized protein. Reduce the carbs massively, no carbs (or at least no potatoes, rice, pasta, bread) after lunch is v good advice.
Don't eat too much fruit either. Some is good but not masses.
Also NO BOOZE. NONE.
Good luck, you can do it, and I promise you that if you do you will feel FANTASTIC on the day. Have a lovely time.