How to get re hydrated after being dehydrated?

I went to emercency the otherday with strep and i was getting dizzy spells. i was dehydrated. how do i get re hydrated. im 13 and weigh 80 pounds. how much water do i need to drink to be re hydrated. my blood sugar was also low cuse i hadnt eaten hardly anything all day how do i get that back up? and how can i stay healthy after the step is gone causeive been sick a ton this year and im sick of it