Pack in the fags and the white lightning on a Friday night is a good start.
Joking apart the conventional advice would be to think longer term, improvements tend to come gradually, and if you did some other runs in the week, some slower, and some shorter runs of a mile or so but faster, your 5k time would come down but perhaps not necessarily over a week. Do your parents run? have you asked a pe teacher at school?
As for Saturday, early night Friday, up bright and early and have a light breakfast, warm up like Cougie said and go for your life! You could ask the volunteers if they know anyone who consistently runs 23 mins or so and pace yourself alongside them...if it is anything like the parkrun here they will probably love to help you achieve your goal.