A Spontaneous Fast

So I've decided to try a fast.

I've never tried doing one before as I have kind of been a bit cynical towards them but I thought I might as well try it to see what it's actually like for myself.
I know a bit about fasts but not much. I've already had 70 calories today which I can easily burn off when I got to the gym later but it's advised apparently according to some sites that you should have fruit juice as well to start off with in a fast which I might incorporate with a smoothie.

I'm starting today I hope I can do this. I'm just worried that if I do succeed that I might put on weight afterwards so if anyone has any advice for me it would be appreciated :).
x