First of all, just your thought/your being led to do this carries a huge blessing, and Grace that you can access to keep you strong.
Long ago I had a brief encounter with street drugs, and could well have gotten entirely lost. Remember that you are not what goes into your body, unless you make it so--and usually that takes time: years of bad habits.
The body and Nature are immensely forgiving. more than most of us know. You are young and have all the potential in the world ahead of you. I suggest you look into all the good fasting literature that's on the "Soil and Health Library".
I like Purinton...he has a dark sense of humor. Shelton and Bass (who has a website) are also favorites. I prefer the older guys to the new books--which strike me as a lot of hype and so on. (I just read the Pleasure Trap and found it ho hum. Stick with the old works. Also look into something called "The Emanation Body". A short Ebook. It has a great vision, in my opinion, of a way to live, after a water fast.)
I started with a 29 day fast. You can look into my posts and maybe find something to help you.
best, Chiron