You will find a lot of nonsense and misinformation on the WWW concerning fasting: it isn't necessary or essential to have juice for a week beforehand either.
You can just begin your fast when you feel the need, with rest and a stress-free environment.
Best to have done a little research from this forum first I would think, and esp the posts marked with red "R's". The Q & A section as well would be helpful to you.
A brief description of your medical history would be helpful, as there can be some contraindications, but these are few and far between.