Water Intake a Day? Nutritionist help? Biking Q's?

[Seb H]

I am going to bike 20 miles on monday and I'm nervous that I may dehydrate. I'm not sure how much I'm supposed to drink a day but I do know that I will be exercising while I bike. I weigh 93 pounds, 5 foot 1, I'm 15 ( I know I'm small). Also we will be leaving around 9 what time should I eat breakfast and how much should eat and what should I eat. Should i bring snacks