Hi Chloee and welcome:

The way 5:2 works is you go to bed, get up, eat 500 or fewer calories during your waking hours, go to bed, get up and eat to your TDEE or less – twice a week. On all 7 days of the week you can eat foods you want to eat – there are no forbidden foods, or foods you have to eat. For the first month or so focus on doing your diet days correctly (500 calories or less) and most everything else will fall into place.

Here is what TDEE means: http://thefastdiet.co.uk/forums/topic/tdee-for-the-curious-or-why-dont-i-lose-weight-faster/

And here are some tips for beginners: http://thefastdiet.co.uk/forums/topic/warnings-to-newbies/

Good Luck!